sloppy -

Putting something heavy - a pizza stone, a good-sized cast-iron skillet, etc - in your oven & pre-heating it adequately (20 min - half an hour) will help with temperature fluctuations in your oven.

As for time/temp in the oven, I've always done 325/5 minutes. My oven doesn't have glass on the door; I can't see inside, so I don't know anything about vapor rising.

As far as monitoring the temperature of the alcohol - don't bother. 151 is ~25% water, remember, but still, it cannot get hotter than ~170f - it evaporates at higher temperatures, just like water can't get hotter than 212f (yeah, at sea level, if it isn't under pressure, blah blah blah).

It's important to keep the residue that accumulates on the inside of the cooking vessel washed back into the alcohol. I use one of those little silicon "spoonulas."

The properties of your cooking vessel affect things, too. I usually use a 2-cup pyrex measuring cup, & the alc/bud mixture simmers. When I've done it in a 1-cup mason jar (thinner glass, taller, narrower, less surface area), the mixture boils much more vigorously.