then which monthly debt payment should you calculate?
I think the only legitimate answer to this question is, "it depends" (on the context). Why are you doing this calculation and what else is involved?
Generally, in a financial sense, debt means money a borrower owes a lender - it's a deferred payment, usually including interest, for some value (usually money, i.e. a loan) that the lender has already provided the borrower. So, in the strictest technical sense of the definition, every dollar that goes through the credit card is "debt." However, if you're asking about debt in a specific context, the answer may vary slightly.
But that doesn't automatically mean that all the money you owe, or all money you spend, is debt. In your example, if you're asking from a personal budgetary perspective, if you're "racking up" $500 every month on a credit card and always immediately paying it back, in the grand scheme, you're essentially just pumping regular expenses through a very short term loan. It's probably even an interest-free loan if you're paying it back quickly enough. From a budgeting perspective, many people would not consider that debt, but would account for that monthly $500 as an operating expense - with the differentiating factor being that "debt" means "things you're paying interest on" - which is an important differentiation from a personal budgeting perspective, since most people doing budgeting have a goal of paying as little interest as possible. So, defining debt as "things I'm paying interest on" helps filter out the white noise of using a credit card as an interest-free tool for deferring payment on regular expenses from cases where you're incurring true debt that you'll have to pay interest for.
Meanwhile, from a credit reporting perspective, the thing that matters is what your credit card issuer reports to the credit bureaus each month - which will be the balance on the card on that day, and whatever your minimum payment is. Depending on the rhythm of when you rack up that $500 balance through the month, when you pay it back, when your statement is generated, and when the bank reports to the bureaus, the balance reported may or may not actually represent that total $500 amount - it will likely be less than that.
Finally, from a debt load calculation perspective - i.e. a new lender trying to determine your DTI as part of you applying for a new loan - the thing that matters is the minimum required payment that was reported on your most recent credit report. In your scenario, a lender trying to determine your eligibility for a new loan would consider your monthly debt load to be $25.
So, depending on the context, you may get a slightly different answer to your question.