So, first, most rules of thumb are not designed for folks focusing on FIRE. But second, boy, I don't even know how to come up with a rule of thumb for things like hobbies -- I mean, if your hobby is hiking, you might spend $100 on hiking boots every couple of years; if it is, say, racing Mustangs, that is a whole different animal.
Best you can do is, I think, try one budget and live it for a while, and then see if that needs adjustment. While you're still in the accumulation phase, you want to start your budget with sufficient savings to get you to FIRE by your target date; then you can split the rest between your needs and wants. So once you know your net after savings and your other fixed expenses, allocate X to hobbies, and see how that feels. Adjust, rinse, repeat.