Boil-off vs Boil time?

A post in another thread prompted a question:

Problems with efficiency have made me think about a larger pre-boil volume, but my time is precious, and I don't really want to extend my boil length. I have noticed that my burner adjustment is very sensitive and I have experienced vastly different boil off rates due to this.

So, my question is: Has anyone ever played with the maximum boil-off percentage possible with out boil-overs? Obviously this depends on batch size and kettle size, but I usually have quite a bit of extra headspace (7-8 gal pre-boil in 15.5 gal keggle).

Any insight?

Thanks.

Batch size is a factor. When I have 13 gallons in the keg, I have to be careful the whole boil. If it's a 5 gallon batch, which I pretty much don't do anymore, you can crank the heat. I have boiled over with 7 gallons in there though.