It's hard to gage the actual values on the API because they use shades of colors which give you more of a high,low ok sorta answer. I'm not a nut about the numbers and am more into stability then .01 accuracy but I do like to know the actual number.
i use api for ph ammonia nitrate and nitrite. the saltwater master kit 16$ cant go wrong with it. used it a handful of time when i first started the hobby 3 months ago and never used it again. i really dont see a need for it but if you are starting out i guess it;s good to have. i use redsea for alk cal and mag there is a kit for 45$ for the 3 which is very good. about 60$ in tests not horrible.