I unwrap the string, grab the ends, and find the middle, then I go out ~20cm from the middle and cut there.This way I have 4.80 en 5.20 pieces.
5.20 for the mains has never been too short for me, but I'm thinking of adding another 10~20.
I always throw away of 40~50 from each mains (sufficient for the last pull), and about 80 after the cross tie-off. Most of the time 10.5/11kg 23-24lbs jobs, but even at lower tension (which should lead to less waste) I never come up short.
If you do come up short (had it happen once on one of my first stringjobs and I used the shorter string for the mains but can reach the last tube with say 10cm to spare, you can use a leftover/old string and a flying clamp to join them and pull that. Obviously not an ideal situation, as the knot is quite an ordeal then, but it worked for me this one time.
I use the 20cm as I saw that on a video on this forum when I started stringing, but as the crosses always have a lot to spare I'm thinking of using more on the mains.
It would be even better for me to not use 10m ready to go strings, but just a 200m reel and cut 9~9.5m as that would be enough, and thus spare some string. I did this once but the hassle with cutting the 200m reel to parts is not worth the savings to me.
I normally measure 8 racquet lenght plus 1 handle length for the mains from the pack, the rest for cross which could be ~6.5 - 7 racquet length for crosses.
If it came short at the end, starter clamp can be used as bridge between racquet & tensioner.
It really depends on what tension is the job, type of machine you use, how far the tensioner away from racquet, so some will need more string than other. Unless you are using 200mm reel to squeeze in extra jobs, otherwise I found that 8 length plus 1 handle for mains & remaining for crosses will be sufficient enough