Finishing the locked Brummel

[seawolf]

Hello all.
Brion. Thanks for your latest reply on my post.
Possibly when I put a lot of tension on the life lines, I will gain a couple of inches in the process.
Is it a common practice to pre-stretch the HM lines before attempting a splice, especially when you are talking long lengths as in stays and shrouds?
Or is this measurement something riggers compensate for through experience?
Possibly both?
And and at what point would you be certain to have stretched enough for proper measurement thereafter?
Thanks.