Forestay Tension

[Brion Toss]

Hello,
First we need to be sure that you do in fact have too much luff sag. How are you measuring it? It is very difficult for an uncalibrated eye to estimate sag with any accuracy, as the max deflection is about 2/3 of the way up the stay. Once you have an accurate measurement, compare it with what your sailmaker was counting on for sag when cutting the sail. If there's definitely too much, then we need to look at other things.
First, that dead jibstay. Is this a Selden furler? If so, a turnbuckle is an option, but the Swedes tend to be so arrogant about the as-installed length that they don't put them in unless forced to. Could be that they, or someone else mismeasured the stay; changing the length is a real can of variables.
Is this a deck-stepped mast? If so, shortening the forestay will, as you suggest, likely not make much if any difference.
The backstay has to have some effect on forestay tension, so if you are positive about the amount of sag, go for as much as 20% on the uppers, and then add in some backstay. But, as you might have guessed by now, I'm a bit skeptical on the amount of sag you are experiencing.
Fair leads,
Brion Toss