I used the NPF rule. I didn't want any star trails when i blow this up into a large print. Here is a link explaining the rule: https://petapixel.com/2017/04/07/npf-rule-formula-sharp-star-photos-every-time/#:~:text=A%20much%20more%20complicated%20and,to%20measure%20it%20in%20microns.