Depends on the rig. On the typical masthead rig the forestay tension is controlled primarily with the backstay, and somewhat with the mainsheet and vang. Hopefully the backstay has some kind of easy adjustment provided.
You need just enough backstay tension to 'get the job done', and no more. Helpful, huh? That's what the pro's say. Or a few pounds shy of popping the bulkhead tabbing.
Ur, not quite enough to drive the mast
thru the keel
are cut the accommodate the expected forestay sag. If it's flopping around it's probably too loose. Big boats have a hydraulic gauge to set the tension.
Sorry, there is no good answer. Take your sailmaker
out for a sail. She will know.