Yeah, you can look at NPK, if you're a biologist or a chemist, IMHO. I don't tend to look at that...unless I start having troubles. I check to see what growing stage the nute is for and I use it then.
And as for using something that's not for MJ, don't do it. While you might find nutes that are cheap, they generally suck and they're just not going to give you the smoke that you want and the smoke that you deserve, especially with all of the time that you have probably put on your grow room.
Another thing to keep in mind, every MJ nute company has their own NPK opinion and tests to back up their opinion. So, for my money, it's best to try out a few different products before you settle on the one that works for you. I've found that Advanced Nutrients seems to have the most extensive research - but that's just my opinion. You have to check out companies on your own.
Yes, you can ask on the boards, but everyone is going to tell you something different. And that's not going to help you at all.
Time for you to do your research!