Before you can work out a budget, you should first work out what you are spending and on what.
Sit down with your bills and bank statements for the last three months. Sort by
Periodic mostly stable expense (electrical, car payment, rent) (sub-sort by period; monthly, weekly, yearly)
Periodic stable income (sub-sort by period)
For each category, annualize it. So the weekly bills get multiplied by 52, the monthly by 12, the bi-weekly paycheck by 26, etc.
Cash withdraws not accounted for count as an irregular cost, for now.
For the irregulars (which can involve food unless you subscribe to it), take the period you are calculating their costs over, and scale it to a year. So 3 months means you multiply by 4.
Insert this all into a spreadsheet. You will now have an idea what your annual costs and expenses are and rough categories.
You can sub categorize them. So, rent is a repeating regular expense; you can give it its own category instead of clumping it in with other bills (like electricity).
Some categories may be bigger than you realize.
It takes effort to save money. Some areas are easier to save money on than others. Some areas are more vague, like "cash withdrawl".
But you now have a start.
Can you afford increased rent? Well, look at your costs. What happens if you boost the cost of rent? Would anything else change in cost?
Can you afford a kid? Find someone who has an estimate of how much it costs to feed, diaper and clothe a kid. What happens to your budget?
Can you afford savings? Find a substantial cost. Is there any way to reduce it?
What is going on with all that cash? Commit to getting reciepts for every cash transaction for a month. Store them in a special place on your person. Every week balance your "cash book" by recording your reciepts and your cash in/cash out.
After a month, do an in-depth analysis of what cash you started with, what cash you withdrew from the bank, what you ended up with, and what your reciepts where for. (There will be money unaccounted for; this method attempts to bound how much money that is)
How does that compare to your 3-month cash usage history? If it lines up, you have evidence it is typical. Identify expenses you can save by eliminating or replacing.
For something like a Car, you can look at your AAA/CAA/Consumers magazine equivalent, and they have information about total cost of ownership of Cars. Or use federal expense per miles. This avoids under-costing depreciation and maintenance, which is really easy to do.
The goal of budgeting is knowing where your money is, and will be, going, and where it is, and will be, coming from. Once you are aware of that, you can make deliberate decisions to change where your money goes with knowledge of what it changes.
The rules of thumb, like X% on food and rent, are just there to give you an idea if you are doing something ridiculous for your social class.