Here is a fertilizer calculator.

Full strength for most fertilizers is usually around 120ppm nitrogen, but many opt for a lower dose like 1/4 strength, or 30ppm N. For your fertilizer that would be about 1/2 teaspoon per gallon. But there is so much excess phosphorus and potassium in that formula that you will want to be sure to flush regularly in order to avoid fertilizer burn. I would really recommend a fertilizer that is more in line with plant requirements. Most plants absorb N-P-K in about a 3:1:2 ratio, which means your plants are only capable of absorbing about 20% of that 5-50-18 formula. The remaining 80% just unnecessarily increases the concentration of total dissolved solids, bringing you that much closer to burning your plants.

If used sparingly, and with regular flushing, it should be ok. Just be careful.

Also, the only real benefit of Superthrive is as a rooting hormone, so I would only use it on cuttings.

Use about half strength until your plants are accustomed to being fertilized on a regular basis and gradually increase it to full strength.

Superthrive is more than a rooting helper. This is just a single aspect for the sues of Superthrive. Add a few drops per gallon of solution. A little will do the same as a lot. So save and use sparingly.

Here is a link that might be useful: Superthrive label

Does anyone really take that label seriously? I mean have you read it? It makes me think of this movie quote.

Superthrive will help with rooting since it contains the plant hormone NAA, but the rest is just unsubstantiated claims, most of which don't even make sense. If people choose to use it on rooted plants, it probably won't hurt anything in small amounts. My guess is that it's mostly sugar water. But I like to think that most people can see this "miracle tonic" for what it really is, a glorified rooting hormone.

Sorry if I sound a little harsh, but Superthrive is one of those topics that drives me a little crazy. This thread has an article that discusses a side by side test done on Superthrive if anyone would like to see more.