Whether you use the tax assessors numbers or add up the rooms and multiply by 1.3 (a fair estimate) their will be inconsistencies. This is why you need a large data sample; making a one to one comparison will be highly vulnerable to false correlations.
Most homes listed in the MLS for Wilmette don't even include the square footage. Condition, location, lot size and age are more important and make comparing homes on the basis of square footage a waste of time. Find a home you like based on these other criteria and you'll be much happier in the end.
To get a ball park idea, measure all the rooms and multiply each each room length x width to get the square footage of each room. Add all the room sizes together then multiply by 1.3. This will factor in the spaces such as halls and bathrooms. The result will give you the approximate square footage.