For race 5 minute count-down we always use B&G H2000 timer. It's very easy to re-synchronise it on 4 or 3 minutes left, if You missed 5 minute signal.
During the race we tend to compare time differences between main competitors in each mark we can. In this case I use my waterproof Samsung B2100
cellphone't stopwatch function. It has function to measure many "laptimes" in order to compare you against nearest competitors.