FWIW:
Your number one defense for a "bad" test (I.E. pH, KH, GH, NH3, NO2, NO3) is a water change, which costs nothing but the price of water, the cost of a bucket, and a length of hose. If you don't already have these things, then why are you keeping fish?
Also, you shouldn't add anything you don't "NEED" to. Don't adjust pH. Precision is far superior to accuracy in this case. My planted tank runs at 6.2 pH. I never adjust to raise it. My cichlids run at 8.0. Only thing I ever did to adjust it was add a few pieces of dead coral and some aragonite...when I set the tank up. Due to high CO2 content in my tap water, my pH out of the tap fluctuates wildly until I get it in the tank. Once the CO2 offgasses, it stays put.
KH, GH, etc...Unless you are keeping KH/GH sensitive fish (Discus, some cichlids), you shouldn't really need to worry about it. Most of the fish you get are already closely acclimated to a KH/GH shift (and likely a pH shift) from their naturaly environment at your LFS (which in most cases has water similar to yours).
Why are you concerning yourself with additives? Like I said, precision > accuracy.
That being said, if your fish are dying, I'd say 95% of the time, it is NH3, NO2 or NO3 related as far as testing goes (disease withholding). If you don't test for these (all part of the master test kit), then why bother? The fish will suffer, become immune weakened and suseptable to disease they could normally fight, and die. Without test kits, you will likely never know why, or how you can correct it (water change....your best defense against disease and death in fish). Likewise, you want to test pH and KH/GH so that you have a precision point, not an accuracy point.
These are the reasons you need a test kit. Not so that you can get accurate 7.0 pH.
Likewise, without knowing your levels of toxins and pH, you are more likely to spend money on snake oil, and medications to correct issues that you are guessing on. With a test kit, you can readily determine what the problem is if it is toxin related.
HTH explain why you need a master kit.