I bought some 15-5-10 a couple of months ago to use on my new lawn. So far, I've spread it once on the front and twice on the back because it's growing out of control and burning through the nutrients (different type of bermuda)

I need to refertilize this weekend but I don't want to add any more P or K to the soil so I went hunting for some different fertilizer.

The bermuda bible recommends something with a very high N content like a 39-0-0. I couldn't find anything approaching that value. I did find some 21-0-0 though. But since it's all laid out in a percentage can't I just follow the formula in "the bible" and still come out with the correct amount for my lawn? In theory if I spread roughly twice as much I would wind up with the equivalent of 39-0-0, right?

I just want to make sure I'm not missing something.