Reducing Pre-boil grav

Hi all, last night I had a problem that was unusual for me. My pre-boil grav was on the high side and my volume as it turns out was a little on the low side. In retrospect, I should have added a little water before I started the boil to compensate. I guess my question is this: is there any way to know how much water to add, or do you just do it a little at a time and keep pulling gravities?
Also- I did a step mash in the kettle for my heffe last night, and wow did that do wonders for temp regulation. Much better than my 10 gall got which I even used to pre-warm.

volume * SG = total gravity points

Therefore,

volume = total gravity points / desired SG

If you collected 6 gallons at 1.050 and wanted to add water to make it 1.045...

6 * 50 = 300 gravity points

300 / 45 = 6.6667

You'd need to add .6667 gals water.

Wow thanks Weston, just what the doctor ordered!