Homebrew PCBs

Hi guys,
   My two Stouffer 21-step step gauges arrived the other day, and I'd like to 
use them to figure out an exposure for a positive-acting precoated photoresist 
board. I've had a look at the instructions on Thinktink's website, which are 
about as clear as mud and seem to be for their own (negative acting) photoresist.

   Can anyone explain how to do this? The laminate I was going to try and 
calibrate is some really old Maplin "Economy Single-Sided SRBP Photo-Etch 
Board" that one of my friends wants testing. Frankly it saves me digging into 
my own stocks of (FR4) board...

   Also, does anyone know how the procedure for setting UV exposure levels 
differs when using positive photoresist?

Thanks,
-- 
Phil.
ygroups@...
http://www.philpem.me.uk/