I would calculate and add just like a normal mash n sparge. Then when you need to do a step, use the already adjusted sparge water. Am I guaranteeing it will be 100% accurate right down the line? No. But it would be very close and has the least chance for failure compared to a bunch of convoluted calculations. Remember the mash is staying put. So it wont need much adjusting. But you will have the same total volume as with a single infusion. So ought to work.
Having said that, are you trying the steps just to see what happens? I did that very recently and have been publishing the results along the way. Its a fun experiment which is leading me to a fairly strong opinion that with modern malts its hard to beat a single infusion and using the vast malt selection we have as a way to drive flavors, and adjuncts and temp for body control. Dig in though! There's nothing like learning first hand and not vehemently attacking something you've never tried. Its always better to say "I have found" rather than "I have read".
Jim- anything you can perceive in the stepped lagers; taste, mouthfeel, head, etc? I just did an alt, and so far all i can report is theincreased efficiency....96% vs normal 87-88%. wont be kegging for another two weeks, and have only tasted a sample during gravity check.
I do plan on doing a lager this weekend with same step schedule-145F for 30 minutes at about 5.3PH, 158F for 45 minutes at about 5.4-5.5PH.