I normally mash with 1.5 quarts per pound of grain. I calculate my strike water temp using the equation in Palmers book for a preheated tun:

tw= (.2/water to grist ratio) (target mash temp - grain temp) + target mash temp.

I batch sparge, and from what I understand I will have my best efficiency if my first and second runnings are an equal volume. Depending on the total grain in my recipe, which changes all the time, I get different runoffs every batch. For instance, I'm brewing today and my total grain bill is 11 pounds, adding in my dead space and grain absorption my strike water will be 6.5 gallons, giving me a 1st runnings volume of 4.5 gallons, then I'd sparge with 3 gallons to get my 7.5 desired preboil volume.

My question is this:

Can I just adjust (reduce in this case) my grist/water ratio to make my first runnings 3.75 gallons? How will different ratios effect my beer? My ultimate goal is to get consistent efficiency batch to batch, will my extraction be all over the place if I change up the grist/water all the time?