There are only 2 depths to lake fishing. The top few feet, and the bottom few feet. In the rest of the water, its sheer luck that you will catch anything.

Are you sure that your reel does not have interchangable spools? If so, you can probably buy a spare spool and load it with your sinking line.

If I had only one line to fish, I would fish a floating line, and stay off the lakes in mid-day, or fish the dingier ones.

One additional book that I just got is "Stillwater Trout" by John Merwin. One other thought is that constrained to a floating line, you might think about fishing rivers and streams for trout. Fly fishing lakes is a masters level class.