I just wanted to mention a system I’ve been using for website quote calculations & an interesting new feature I discovered. Its a Wordpress Plugin called Formidable Pro & it does all the usual calculations (price per window + a percentage per floor etc etc),… but these systems always have a limitation - they rely on a logic chain to calculate the price. I’ve been using this system for a while now & using the standard calculations.
For example, if you have a checkbox to “include screen cleaning” the system can add a fixed amount, or add a percentage to the total fairly easily - but if you want to adjust that amount/percentage depending on the size of the property (eg it adds 50% to the price for larger properties but adds 65% to the price for smaller properties to compensate for setup time) then things start to get very complex.
So this system lets you create an array - you give it every possible combination of options and just enter a final price for each combination. Zero calculation - just the price you’ve set which matches the options selected. No complex math, dead easy to adjust pricing. The system lets the user select their options and retrieves the matching price from the array… simple!
You can even enter multiple prices - one for each service, let the customer select which services they want, and total it up for them. 
Its in the “Drill down to a final value” section of this page - complex reading I know, but a brilliant system!
