Quick Estimate for sqft?
 

I have spent the last 30 minutes trying to find a thread from several years ago on figuring sqft of drywall, I couldn't find it and hope someone will let me know the figures. The formula took sqft of floor space times 3.8? for a 8ft ceiling I think? If anyone knows I am looking for 8,9 and 10' numbers. I have been estimating about 3 houses a week now for a month and wanted to recheck my numbers. I have been using 3.8 for 8', 3.95 for 9' and 4.1 for 10', how far off am I?
I am not trying to get exact numbers, just a good estimate, seems like there are a lot of tire kickers this time of year.

Thanks