Trying to figure out how much money you have available each day sounds like you're making this more complicated than it needs to be. Unless you're extremely tight and you're trying to squeeze by day by day, asking "do I have enough cash to buy food for today?" and so on, you're doing too much work.
Here's what I do. I make a list of all my bills. Some are a fixed amount every month, like the mortgage and insurance premiums. Others are variable, like electric and heating bills, but still pretty predictable.
Most bills are monthly, but I have a few that come less frequently, like water bills in my area come every 3 months and I have to pay property taxes twice a year. For these you have to calculate how much they cost each month. Like for the water bill, it's once every 3 months so I divide a typical bill by 3.
Always round up or estimate a little high to be safe.
Groceries are a little tricky because I don't buy groceries on any regular schedule, and sometimes I buy a whole bunch at once and other times just a few things. When groceries were a bigger share of my income, I kept track of what I spent for a couple of months to figure out an average per month. (Today I'm a little richer and I just think of groceries as coming from my spending money.)
I allocate a percentage of my income for contributions to church and charities and count this just like bills.
It's a good idea to put aside something for savings and/or paying down any outstanding loans every month.
Then I add these up to say okay, here's how much I need each month to pay the bills. Subtract that from my monthly income and that's what I have for spending money.
I get paid twice a month so I generally pay bills when I get paid. For most bills the due date is far enough ahead that I can wait the maximum half a month to pay it. (Worst case the bill comes the day after I pay the bills from this paycheck.)
Then I keep enough money in my checking account to, (a) Cover any bills until the next paycheck and allow for the particularly large bills; and (b) provide some cushion in case I make a mistake -- forget to record a check or make an arithmetic error or whatever; and (c) provide some cushion for short-term unexpected expenses. To be safe, (a) should be the total of your bills for a month, or as close to that as you can manage. (b) should be a couple of hundred dollars if you can manage it, more if you make a lot of mistakes.
If you've calculated your expenses properly and only spend the difference, keeping enough money in the bank should fall out naturally.
I think it's a lot easier to try to manage your money on a monthly basis than on a daily basis. Most of us don't spend money every day, and we spend wildly different amounts from day to day. Most days I probably spend zero, but then one day I'll buy a new TV or computer and spend hundreds.
Update in response to question
What I do in real life is this: To calculate my available cash to spend, I simply take the balance in my checking account -- assuming that all checks and electronic payments have cleared. My mortgage is deducted from my checking every month so I post that to my checking a month in advance. I pay a lot of things with automatic charges to a credit card these days, so my credit card bills are large and can't be ignored. So subtract my credit card balances. Subtract my reserve amount. What's left is how much I can afford to spend.
So for example: Say I look at the balance in my checkbook today and it's, say, $3000. That's the balance after any checks and other transactions have cleared, and after subtracting my next mortgage payment. Then I subtract what I owe on credit cards. Let's say that was $1,200. So that leaves $1,800. I try to keep a reserve of $1,500. That's plenty to pay my routine monthly bills and leave a healthy reserve. So subtract another $1,500 leaves $300. That's how much I can spend.
I could keep track of this with a spreadsheet or a database but what would that gain? The amount in my checking account is actual money. Any spreadsheet could accumulate errors and get farther and farther from accurate values. I use a spreadsheet to figure out how much spending money I should have each month, but that's just to use as a guideline. If it came to, say, $100, I wouldn't make grandiose plans about buying a new Mercedes. If it came to $5,000 a month than buying a fancy new car might be realistic. It also tells me how much I can spend without having to carefully check balances and add it up. These days I have a fair amount of spending money so when, for example, I recently decided I wanted to buy some software that cost $100 I just bought it with barely a second thought. When my spending money was more like $100 a month, lunch at a fast food place was a big event that I planned weeks in advance. (Obviously, I hope, don't get stupid about "small amounts". If you can easily afford $100 for an impulse purchase, that doesn't mean that you can afford $100 five times a day every day.)
Two caveats: 1. It helps to have a limited number of credit cards so you can keep the balances under control. I have two credit cards I use for almost everything, so I only have two balances to keep track of. I used to have more and it got confusing, it was easy to lose track of how much I really owed, which is a set up for getting in trouble.
- Never ever ever rely on what the bank says your balance is. There are always transactions that haven't cleared yet and so don't show up in your bank's calculation of the balance. You know you wrote those checks or scheduled those payments. Keep an accurate checkbook. Every now and then I see how much money the bank thinks I have and I always find myself saying, "I wish I really had that much." Well, if you're talking about keeping a spreadsheet and tracking expenses, you're probably doing this.