Now correct me here... My current setup I find I boil off about 1.5-2 gallons liquid over a 60-90 minute boil. So should I consider my pre-boil volume more like 7.5 gallons instead of 6?
Yes, figure out close to what you boil off and what you want in the primary. If you want 5 finished gallons after fermentation plan on 5.25-5.5 as you will lose volume to trub.
So that right there keeps my kettle from over filling, but 4.23 gal sparge water isn't probably enough to achieve a good efficiency since 17lb grain* .5gal/lb sparge = 8.5 gal sparge water needed.
7.5 gallon pre boil volume
17lbs x 1.25 qt/lb = 5.31 gallons strike water
17lbs x .12 absorption = 2.04 gallons lost
First runnings =3.27 gallons
7.5 gallons-3.27 gallons=4.23 gallons required to sparge (There is no necessary ratio to calculate. The amount of sparge water used is the amount needed to get your pre boil volume after 1st runnings, nothing more. Pretty simple.
Now a couple things: Because you would be pushing the limits of your kettle you should consider an anti-foaming agent like Fermcap-S which will reduce/eliminate boil overs.
Losses. there are always some losses in the tun and the kettle that need to be taken into account when calculating your volumes and every system will be different, There will always be a little liquid left behind somewhere.
Ability to boil full volume. It is quite difficult to bring 7.5 gallons of wort to an adequate boil on a indoor stove so keep that in mind. Most people move boiling outside on propane burners. Those that stay inside sometimes will split the wort into two separate kettles to use 2 burners and then combine the wort into the same primary. Some will also span 2 burners with one kettle if the stove/kettle allows for this.
a 10 gallon mash tun can accommodate up to about 23# of grain for some really big beers although my experience is mash/lauter efficiency will suffer a bit as the grain bill increases in weight.
Keep in mind that I used 1.25 qts per pound for the mash. This would be on the lower end. Most that I know, me included go 1.5qts/lb so that will change some of these example numbers.