I use the mfj 989 B, have for about ten years. The idea is to use as
little inductance as possible. I mark down the number of turns per
band. From 1 end to the other is about 40 turns. I have gone past the end
a few times. This demands that I open the tuner, lift the roller bar, and
slide the roller back so it is on a coil turn. Try to fit it on a turn
rather than between turns. I clean the roller and the coil about once a
year. It's a great tuner, and more efficient than a coil tap. It also
tunes anything.