Hello,
I commend you for your caution. We have a chart we could send to you, showing typical deductions for given lengths and loads (contact
maria@briontoss.com). The load part is important, as how much you deduct is largely based on expected tune. On your boat, for instance, tuning stretch for the uppers might approach 3/4" or so, but only if the wire is the right size for a 15% to 20% tune. Constructional stretch will always be the same for a given length, but this is usually a teeny deduction.
Fair leads,
Brion Toss