Homebrew PCBs

So get a PCB with film, cut it into strips the width of your step
gauge, and do a binary search of your timer's range ;-)

Really, if you have a step gauge, you don't need to ask any more
questions - do ANY exposure with the step gauge for some time, and
based on the results, calculate the "right" time.  Since it's
logarithmic, any reasonable time is an acceptable starting point.