Shift light for 5 cyl?

[Dwight & Tina Varnes]

I've been wanting a 'shift light' for my CGT, since the Momo steering
wheel effectively blocks the view of the most important portion of the
tach. Autometer makes a nice, unobtrusive little one I could stick
somewhere for my track days and solo events. The problem, as always, is
they are made for "4, 6 or 8 cyl".  Since they are triggered by ignition
impulse for a single plug, I'm wondering if I can't just set it for 4cyl
and add 25% to get the rpm limit I'm looking for?  (i.e. 4 cylinders =
100%, therefore one more would be another 25%).
Autometer unfortunately uses individual 'pills' to set the activation
limit. VDO uses and adjustable knob to set the level, which would
probably be better. I'm just wondering if there is a reason this would
NOT work before I spend $85-150 on one.

Wonder if I could just buy the trigger knob and use the upshift light
instead???

Dwight Varnes
86 CGT
http://www.mongrelmotorsports.homestead.com