I am working on a ring that adds some lightning damage to damaging spells that adds more damage per spell slot level used but I don't know how much it should be. so, any ideas? (very rare, Requires Attunement)
Look for similar abilities (i.e., abilities that add damage to spells or attacks). Do they have costs or limited uses? Then your item should probably be weaker if it doesn't.
Consider how often it gets to trigger. If it's any damaging spells, then it should be less than if it just boosts lightning spells.
Most importantly: it's ok if you don't get it precisely right. You can always say to the players later "that ring I gave you is a bit too good, and I'm tuning it down some", and most people will be fine with it. (And they'll be even more fine with "that ring I gave you is a bit underwhelming, so I'm tuning it up.") You can also make the encounters tougher to compensate, although it's less than ideal.
I am working on a ring that adds some lightning damage to damaging spells that adds more damage per spell slot level used but I don't know how much it should be. so, any ideas? (very rare, Requires Attunement)
nvr mind
well see here I made it kinda busted at lv ~6-9 but its fine https://www.dndbeyond.com/magic-items/11143074-fibonaccis-ring-of-storms but also its planned to be a boss drop