If you know what depth you want to start your scaling at, you can select that as your max value instead of 0. For instance, -10000 min and -100 max, then a bit of feathering. May have to experiment to get it where you want, but feathering in Wilbur, based on my experience "shrinks" the selection instead of expanding it. Waldronate can, I hope, verify that assumption, or at least correct it if I am wrong.