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.