John Popelish sez:

Higher zener voltage means faster current ramp down. But
you will probably have to go quite a bit higher to see much
difference. The resistive drop of the coil is already
starting the ramp down with a 42 volt reverse voltage. But
that drop falls as the current falls, so the zener is really
there to speed the tail of the process, unless its initial
voltage is on the order of the supply voltage. So you might
consider one as high as 22 to 39 volts. But then I would
look for a 1 watt unit, to handle the power pulse that will
end up more there than in the coil resistance. But you
should definitely see some decrease in the power down time,
to about 37% if what you will get from a 4.7 volt zener if
you switch to a 33 volt one. So you can see that the turn
off time is not dominated by the zener till its voltage gets
near the supply voltage. But increasing the zener voltage
drop helps.

Seems we're creeping back up toward the original 47v zener (although it was
connected across the FET, not the coil). Any advantage to simply using
another 47v part along with the rect. in the configuration you recommend? Is
this a case of "bigger (v) is better"?

Thanks again,
--
John English