Michaels question is fine if the moisture content of the bread is known. But sadly this is where it all goes wrong. To get the precision that Michael wants requires for the bread to be of a known standard and consistent. Bread that is 2 or 3 days old will brown far quicker.
I’m sure if there was a market for such precision then machines would be available,to analyse the bread and then set the time for perfection. To go with this perfection one needs to be ready to collect the bread as it pops up, spread the butter and apply the jam then eat it whilst still warm. I find it’s better to under do it, then give the last few seconds, grab it and go.
Good luck in your quest Michael. Noel.