Billing
Balance minus anything held by requests in flight is what a key can actually spend right now.
Balance, held, available
The dashboard shows three numbers. Balance is what you have paid in, less what has settled. Held in flight is money reserved by requests that have not finished. Available is the difference, and it is what the next request is checked against.
Money is integers
Everything is stored in nano-dollars as integers. Floating point on a per-token price drifts, and on a 15% margin that drift showed up as whole percentage points, so it is integers end to end and rounding happens once, at settlement.