I have Florin's #4 tester and have checked it's accuracy on test equipment in my electronics design lab. It greatly surpasses the accuracy needed for testing mechanical shutters, as would be expected from a single chip microprocessor design such as this. The device is well constructed for its application/price/functionality and is easy to use, lot of value in this package.
I have turned a Delrin stepped adaptor stand for the sensor and cap for the light source that fits Copal 0 & 1 and dismount the the lenses for LF, test the naked shutters, put the values in a table, and expose according to the measured speeds. For the MF and 35mm, I measure periodically just to check when/if to CLA. I initially bought Florin's tester for portable use, verifying shutters on the road, and have, since receiving it, given up the bench test setup I built and use my #4 exclusively.
Florin is a great guy to deal with and you will not be disappointed with this useful tool.