[RWP] Limiting Project Length In Project Settings

[Indigo]

I've been having a hell  of a time trying to understand what digits are 
expected in file/project settings/limit project length.
I found a cockos forum with someone complaining that all his recordings 
stopped at exactly ten minutes, and the default numbers I found in my 
Limit Project Length edit box were 10 colon 00.000, which makes sense 
for 10 minutes.
Okay, now I realize I need to enter the number of seconds required to 
record a 1 measure loop at a tempo of 120 bpm.
I guess I could change the timeline to hours/minutes/seconds, then hit 
play and listen to the metronome, and stop play at the end of 1 measure, 
then read the timeline to learn how many seconds are required to record 
1 measure at 120 bpm.
Man, it seems like I'm gonna need to recalculate how many seconds for 1 
measure each time I change the tempo!
Can anyone think of an easier method to stop recording at the end of 1 
measure?
Thanks for any help,
Indigo