Well you want to be .04-.09 ish. This is up for very high debate but if you have .01 or .02 you could have diatoms. If higher I doubt you have diatoms.
don’t freak out because there are plenty of systems that run much higher. However starting out just stay low. People will tell you they run really high with no issues and it won’t matter but they probably have a very established system.
With my system I can literally see where my phosphate level is based on when the diatoms start to show up. I just tend to feed a little bit more the next day they die a little bit back.