That's not a stupid question at all. It's a good idea to test when you fill up. That way you'll know the E content of what you're adding to the tank. Pump gas is up to E10. Sheetz E varies a bit, with the biggest difference being from the summer blend to the winter blend in my experience.
I just blend at the pump. Each time I fill up, I test what's coming out of the E pump and run the numbers through an E content calculator on my phone to get the total amounts of E and 93 to make my desired E blend.
In your case, there's a bit more wiggle room since you're not targeting a specific E blend. 1-2gals of E per full tank will be enough for you.