Is there an easy way to calculate an aperture scale for a shutter/lens?

I have my 210mm Kowa Graphic in a Copal #1 shutter with an appropriately marked aperture scale.

Say I want to put a 150mm Kowa Graphic in to try it for a while - is there a way to come up with a scale without sending the whole thing off to Grimes or somewhere? (I'm not talking about the actual little piece of metal for the scale itself - I'll just use a piece of tape or some such and mark it - rather the measurements)

Jim G - don't you add scales to some of those Polaroid shutters?

thanks