Pay your credit card off the day you use it for a transaction. Yes, that exact day.
Forgo the credit free period in exchange for the control paying immediately gives you. Credit cards should be used to make a transaction safer and more convenient, not as a delayed cost mechanism. And if you haven’t got the money, you shouldn’t use the card. The habit we create with paying it immediately, is worth much more than the potential gains in interest. Habits are always more valuable than micro financial gains.
Oh, and by the way, funding your startup on a credit card is a mugs game. Every winner we read about it working for is among a thousand invisible and unreported losers.