first extract and partial mash with rye malt

I'm doing a 5 gallon batch of Belgian Strong Dark Ale using 1.75 lbs. of rye malt and 1.25 lbs. of Special B malt plus 6.6 lbs. of Light LME and 0.25 lbs. of Light DME. I'm also adding 1.25 lbs. of dark brown sugar.

I milled the rye malt and the Special B malt together at the LHBS, as suggested by the proprietor. I plan on doing a partial mash of the grains. How much water should I use and how long should I mash? Finally, how much water should I use to sparge?

Is this a full boil? or is your top off water your 'sparge'?

I'm not sure. In the previous two batchs I've made, both IPAs, I steeped 8 oz. Crystal in a gallon of water then used another gallon of water to sparge. Both of these were added to 4 gallons of water I used to make the wort, boiling for 60 minutes.

In this case, I'm not sure exactly what to do. I'm thinking I should use 2 gallons of water to mash the grains, use another gallon to sparge, then add 3 more gallons to do my full boil?

ya if you have a pot to hold 6gal and you are getting 1gal/hr boil off...