Ah yeah man, my Hanna alk reads .1 low pretty consistently, its just something to keep in mind, I'm much more concerned with consistent results than the absolute number. If you are getting lots of variation between tests, I would suspect a couple things. The most probable is (1) messed up cuvettes, so it might be worth buying new ones. After that (2) inconsistency with your sample or reagent amounts. Buying a single channel adjustable micro pipette should fix this problem they aren't super cheap, but they tend to last and can be used to increase the precision of all your tests. After that I'd say inconsistent testing method, its important to preform the test (this goes for any testing you do) identically every time if you want better precision from the test, and finally maybe you just have a messed up Hanna Checker.
As far as completing the Phosphate test in time, prepare the reagent packet ahead of time, get it opened and shaped into a funnel before you even hit C1. Once you are ready place the cuvette into the checker and when you hit the C1 button start a timer (I use the stopwatch on my phone), give yourself 1 minute to pull the cuvette out and dump the pre-prepared reagent into it. Then shake gently for two minutes (I shake for 1:40 and then swirl/invert for the last 20 seconds to eliminate micro bubbles, but whatever you do be consistent in how you preform the test!!), for a total of 3 minutes since hitting C1. Then place back in the checker and press and hold the button. The key is figuring out how to prepare the reagent packet ahead of time so its easy for you while you are trying to beat the timer. Like I said in my previous post there are guides online you can find. Good luck, hope this helps!