[dhacker]

Denny's point is valid. I typically observe the boil off only for the 60 minutes immediately after the hot break. That tells me the boil off rate not only in percent (for the first hour) but also, the gallon(s) per hour rate.

[tygo]

Right, I boil off about 2 gallons an hour for anywhere from 60-90 minutes.  If I'm boiling for 90 minutes that's 3 gallons of total boil off so to achieve 6 gallons I'm starting with 9 which makes my calculated boil off percentage 22%.

So I'm just wondering what difference that makes if any over a calculated boil off rate of 15%?

[dhacker]

I know that if I was getting 20 to 23% for the first hour, my Sabco Brew Kettle would be jumping off the stand! 

A good roiling boil is what I believe is the important thing. Let the boil off run where it will. using a 15 gallon pot for a 6 gallon batch should allow plenty of capacity. I also feel that as one gets comfortable with their brew rig, they can almost anticipate where they will end up at the end of the boil just by the visual cues going on inside the kettle.