I measure off the length of the pull as acurately as possible. I don't add for sweeps. I add the actual length for rise and drops.
Then add for your desired tails or waste needed at each end, ( usually 5-10 ft.).
If I am unable to get an exact measurement I will add a little more to be safe.....just depends on how I feel about it.
I usually don't take the time to push in a fishtape and measure it.....allthough I agree this is the most acurate way.
I haven't been short on a big pull.....yet. I have, however, had times where I expected 10' of drop-off and ended up with less than 1'.