I don’t think there’s a magic number, though you can start by assuming there’s one. It’s certainly more productive to aim for 0.36 than to have no goal.
The problem with diet as a solution isn’t your 0.4 test result (which I understand was affected by allopurinol) but your 0.51 result. Getting from 0.51 to 0.36 without drugs would be a serious challenge indeed.
One problem with Arcoxia in my opinion is that it’s initial effect is weak compared to Indocid, especially if you only take 60mg. Colchicine in combination with Arcoxia might be a powerful way to end a lingering attack over the course of a day or two but you may find yourself waiting way too long for their effect to really kick in if you take these drugs after an attack starts.