I really think proper depths is not as big a deal as it's made up to be so being limited to only a few lines isn't a very big handicap as you might expect as far as finding the right depth quickly. It might perhaps increase your odds of catchin fish running more lines however.
Last summer I bought my first Rod Pumper from Jim and as I refuse to fish with more than 5 colors of lead core with a 300 or 400 feet of line back. So my starting set up was 100ft main line, 4 colors in-line lead out with an extra 100 feet of mono after that, about 320 feet back. So I figure I'm down about 25 feet max with that set up, not a great depth normnally for Sqaum which is a warmer lake than Winni. However we caught big Salmon all summer and fall with that set up, many times the only line that got a bite all day.
My contention is depth, color, etc. are not that important, if a hungry Salmon or Bow can "see" your spoon, fly etc. they will come well out of thier comfort zone into warmer water and take a bite.
Would I run all my lines at that depth in summer, probably not (definately not) but you can still catch fish at most any reasonable depth, actually I think your odds are better at 25 feet than 100 for Salmon or Bows.
Trust me 20 or 25 feet is just a few flips of a big fishes caudal fin, so don't get too worked up being limited to 2 lines to find the correct depth, I would say 28 to 32 feet will work on most any of our lakes.