Don't let recommendations of how long roosts should be worry you too much. I've seen recommendations on here anywhere from 7" per chicken to 15".
No joke. I have more than plenty of roost space for my 6 girls and it never fails they’ll all be jammed together as close as they can get on one roost. Or they’ll find a freggin window ledge to roost on overnight in the coop. We get in our minds that there are certain EXACT specifications for chicken raising and where as there ARE certain things we should be mindful of, try to put into practice or avoid doing, Chickens are animals and do their own thing. And like I’ve said before- chickens are WEIRD! But from the suggestions given by the experts here I think you’ll be fine.