I have got hold of a snap-on timing light with adjustable timing...but I'm not sure how to use it.
Obviously I can rev the engine to 6000 and check it's hitting the 32 deg mark (or whatever it is), but what is the dial on the back for?
can anyone enlighten me

You set the dial to the desired advance number and time with the TDC mark.

or

You set it to zero and start turning the dial until the TDC mark comes up in the notch and that's the amount of advance you are currently running.

We, here in the colonies, are always glad to help.

The dial on the back shows how much timing delay you have when you
turn the knob and see the top dead center mark on the blower looking
through the timing hole. That is at 3000 RPM's you should have 28 degrees
showing on the dial with the TDC mark lining up.

Hope this helps.

so....for engines without a mark for total advance this would be helpful. Or maybe useful for checking the advance at any given RPM (assuming one can hold the rpm steady enough).

but not really a massive help

That's about the long and short of it.

The dial has "Engine Degrees Advance" silkscreened on it.

For a stock motor, you have the 27* timing mark and the 0* timing mark. Set the light at 0 and look for the 27* mark, then set the light at 27 and check for the 0 mark to double check.

And when you find a motor that has a fan without the 27* mark, you'll be able to help. And if you want to time a carb motor to something other than 27* (for example, 30* full advance) then you can do that.

Definitely handy, you'll use it someday.

Cheers guys, I know it adjusted the strobe to the desired advance
I just couldn't see the point really