Simms, might want to consider a G tech Pro competition unit for around 240.00 US funds.
It has a built in shift light with adjustment digitally, no pills. Also runs on a 4 led system I think it is. First one goes off at set point minus 500, second light comes online at set point minus 250 and then all 4 flash at setpoint.
Best part is all you have to do is plug it into your lighter.
Consider the built in dyno feature, 60 foot times, 1/4 and 1/8 times, lateral G force in all 4 directions and the option to shoot it to your hard drive to analyze runs all a bonus if your going to spend some cash on just a light.
I love my Gtech, lots of fun to play with. 100% accurate?, probably not to the hundredth for timing, but I would say within a tenth. Any incline of any sort throws the built in "gyrometers" into fits for obvious reasons.
best of luck,