charge by Lft or Sqft?

I was wondering about this. So I got to do an estimate and measure the windows. Some are 33"W X 54 H" others are say, 24WX60H ect. I have to spend too much time figuring out the sqft of glass, then amount of lft I have to buy and how much waste I will eat. So What I'm wondering is: Is it better & faster to just figure the lft of the glass (long side), and say all windows fit within a 36" roll, get the total Lft. and price it on what I have to buy not the actual sqft of the glass? Then, would I reduce the price, say for instance if I charged $100 a sqft of glass to say $90 per lft. Disclaimer--These prices are for ease of problem solving only-. Would using lft method make my estimate larger then others who use the sqft method on the same amount of glass? To Lft or not? that is the question!

I figure alot of jobs on linear feet if the width of all the windows is pretty consistant .

If you don't account for waste, your profits will be lame.

linear feet. A carpenter will charge you for a 2x4x8 , even if he only uses 7.5 foot of it

I always round up on my measurements as well. For example, that 33x54 window you have I would cut out of a 60" roll and round it up to count as 15 sq. ft.
Like OT said, if you don't take into account the total amount of film you are using then you are just throwing your money away.