Those estimates given work okay for shallow depths, but the error increases as you go deeper, and a foot can make all the difference to a crappie. If the line is at 45 degrees to the water, the depth will be just above 7' for every 10' of line under water. So at 15' of line out, the depth is closer to 10.6'. Not too much difference, but it could make a difference to the slabs.

I always thought that rod weight ratings were for casting? A rod rated for 2oz should be fine trolling 3oz and a crank, shouldn't it? I mean, if it can horse in a 2lb fish, why would trolling a tenth of that weight have any negative effect?