Hi Craig,
We have no formula for pricing commercial v's residential, however we use a job we do for a local hotel as a starting figure.
It equates to £40 per hour which is our area is a good rate. we would just calculate how long the job would take us and times it by our rate per hour.
The rate you get per hour is very dependent on how much competition you have locally. I would recommend you work on a price per hour rather than a cost per window but other may think differently. :
Sarah