Not sure this solves your problem, but:
Here in Norway we have used the 40/25 play clock for a few years now. No teams have any visible play clocks so the play clock is kept by the R. Here is what works for me with the RFT100 set to vibrate:
I set it with 3 intervals. First is 15 sec, second is 10 sec and third is 10 sec, for a total of 35 secs. What I quickly discovered was that it was impossible to reset it (if needed) and start it exactly when the play ended. So when the play is over I wait for 5 secs and then start the counting down. So when it vibrates for the first time I know 20 seconds have gone. Next time it vibrates I yell '10 seconds', and when it vibrates for the final time, which is a long vibrate, it is time to throw that flag if they are not snapping. And if it is a 25 sec play clock the first vibrate is '10 seconds' and second vibrate a possible flag.