Most older Harken furlers are set with a toggle screw that is exposed at the bottom of the furler drum. Tuning is simply adjusting the toggle screw until you get the proper forestay length; but, the actual tuning tension loads on the forestay are from the backstay.
'Perfection' forestay tuning is when the 'luff hollow' of the jib/genoa (the 'hollow crescent' section of luff material that is cut away from the luff by the sailmaker) exactly matches the 'sag' of the forestay when the genoa is wind-loaded. If this 'matching' is of interest to you (for very ultra-precise 'performance' sailing), I can explain it fully in a follow-up posting.