Anyone?!

any architects here?

I have done calculations before, what exactly are you looking for?

there are 2 methods as I remember. 

it is not as easy to explain in forum discussion. My firm had to purchase their calculation package, which comes with detailed explanation of methods used for calculation and spreadsheet where you have to enter data.

check "BOMA 2010"

basic idea is to have separate square footage of each suite in the building, square footage of support spaces, such as janitors etc, square footage of lobbies waiting rooms etc., separate square footage of stair cases and elevators and there are other factors such as parking and site. after that you are entering this data in Excell spread sheet which is set up with formula to calculate, actual square footage  and rent-able square footage of each suite in the building.

it seems easy, but it is not so strait forward, I would suggest to purchase their package or you are taking too much liability trying to figure it out yourself. they are very specific how to calculate windows, doors and many different items in the building. for example if you have store front windows or standard windows it makes difference when taking measurements.

I was curious where you measure from? Outside face of exterior wall. If a wall is over 50% glazinge then you measure on inside face of the glass. 

Then centerline of any demising walls separating tenants. 

Sq. Footage also includes columns and extrusions correct? 

Restrooms and Corridors are also included in the Sq. Footage.

I feel I am missing something?

outside face of exterior wall seems common in industrial buildings, or to the outside face of an overhang if there is one.  i think they do this in retail sometimes too.  that's something you may need to confirm with your developer or client.  i think BOMA area measurements are usually only for office occupancies.

for offices it's to the inside face of exterior walls and inside face of glass where glass is a dominant surface.  if there is a common corridor and restrooms then you measure to the inside (tenant side) face of walls.  the area of the wall goes to the building common area.  measure to the middle of demising walls between leasable tenant spaces.  columns are included in the tenant's area.