In case you missed it, there is a wealth of advice in the various
tuning guides on the main J/30 page.
Backstay doesn't really factor into rig tune. It should be loose enough to wobble sailing upwind under 10 knots. A quick flick should be all it takes to clear the backstay from the mainsail top batten during a gybe. Adjust turnbuckles so that the bridle blocks, when loose, return to connector plate at the top of the split. Tension using the purchase as breeze increases and the boat becomes over-powered to flatten the main. In a blow, tighten the turnbuckles so the backstay purchase is more powerful. To set shroud tension start with the lowers, then tighten uppers, sighting up the mainsail luff groove in the mast to keep the mast in column.
There should be some play in the headstay once the shrouds are set. It won't be bone tight. It can be difficult to get an accurate measure of headstay length with the rig up. The method described in this
post works well enough for me. You can attach a dumbbell or jug of water to the halyard to maintain consistent tension as you measure. There are two schools of thought on headstay with some always at max and others, myself included, going tighter as the breeze increases. Looks like you have roller furling, which probably means set it and forget it, but 35' 8" is very tight. Could you add a toggle to extend the headstay?
