High phosphorus is better than anything. 10-10-10 is fine but 10-20-10 might be better at encouraging blooms when regrowing from a state of severe dieback.

Yeah, you do not have to have the exact NPK. I would use whatever I have that is close enough (and to me, your 15-30-15 is close to all 10s). But... when you run out and it comes tie to buy more, theeeen I would either look for something closer to 10-10-10 or just use 1 cup of composted manure, organic compost or cottonseed meal. Instead of exactly matching NPK numbers, I find it is probably more important (a) not to exceed the amts on the fert instructions or (b) to even fertilize slightly less than the instructions call for,