Estimating interror size based on Building size?

[Plato Panagos]

I'm curious if anyone can comment on my way of estimating apartment interior sq/ft based on building size. Since Listing agents almost never give sq/ft for apartments buildings what I've been doing to get a rough idea of apartment size is to access the local assessors site and get the building size, I then subtract 10% for common areas and then divide the remainder by the number of units. I'm not sure how accurate this method is and wanted to know what others think. 

I'm not looking to make any detailed assessments based on this just a large, average or small comparison. I also understand assessors definitions can be different but in this area basements below grade are not included.