That's a nicely sized boiler! You should get a lot of productivity out of that beast!
Let's assume that you want a 25 bbl yield to the fermenter, after chilling. Your cooling loss could be as much as 1 bbl, and at the evaporation rate supplied, you'll lose somewhere around 3 bbl for 60 minutes. That means measuring 29 bbl preboil, above the racking port at 200 deg F. [For purposes of this discussion, I'm going to ignore what's below the racking port]
That sounds like a pretty full kettle. I have a 15 bbl system, which fills to 18 to 19 bbl, and has a capacity of 23 bbl. Even with 4 bbl of headspace, it doesn't take long for me to get a lot of foam, if I open up too much steam. Foam Control is my friend!

The main number is that it takes 1150 btus to get 1 lb of steam. A gallon of water is 8.33 lbs and contains 180 btus when it's at 212 deg F. The difference to convert all of that mass into steam is 970 btu/lb. Sum that up and you need around 750K btu, which is well within the range of your boiler.
If the kettle has a very wide geometry, you'll get close to this, but probably not above 8% boiloff. If it's pretty tall, you'll be closer to the 5%. That dials your preboil back by a barrel, maybe two. This sounds more reasonable and it places the cooling loss a lot closer to what you got from BS.
Next post, I'll look at brewhouse yield.