my method might be a bit too "cumbersome" for most, but it works for me.
i do use a stop watch. i just feel without that piece of "technology" you can't practice SF to i's fullest.
at the start of round 1, i press start. at the end of the round i press stop. i record the time, "zero" the stop watch and press start again. at around about 2:30 of rest time i stop it again and "zero" it. i then get myself ready for round 2 and repeat the steps above, until i completed 5 rounds. i then add up all the time i used to do the rounds and that gives me my total time for D2.
two things i've figured out using this method - and both involves the fact that i do the timing myself:
1. i do not rest a full 3 mins between rounds. it's close, but not quite exact - cause i have to stop the stop watch at the end of the rest time, get ready for the next round and start it again. my timing is not always right.
2. because i do the timing myself, my first pull-up is always a second or two later than what i started the stop watch. therefor i always deduct a second from the "stopping" time, to compensate for the time lost between starting the stop watch and doing the actual first pull-up.
i hope the above makes sense and helps.
shout if you need more advice!