I agree hourly rate is a pipe dream when starting out or converting from trad. Speed will automatically come with experience of each job.
I think price per window and multiplying is the best way to charge on domestic work.
Remember that you WILL GET SOME PRICES WRONG !!!! Don't worry I'll bet every on here as windows they do every month and they think
"for **** sake £10 for this ******* place I must be off my ******* head"
The trick is to take the rough with the smooth Try and aim for a set daily amount.
If you price per window you will find that you dislike doing houses that have kids toys / trampolines in their garden or have akward gates etc.
An hourly rate gives you a more even kiel.