You could open a can of worms with this question, but close to zero is generally what you want.
Less than .03 is a common result.
.03 on a hanna meter is perfect! If you got 0.0 your tank would be starved. If you had sps they would be pale in color. 0.0 on a hanna meter is a very difficult level to get to in any system.
On any conventional teskit such as salifert, api, elos, merrick etc. You should always get 0 as a reading. Regular phosphate testkits are useless at low ranges.
Btw i am speaking about the older more expensive hanna meters. I am not sure how accurate the next po4 checker is.
I just got the hanna as well, my phos was 0.03. hmmm gotta get it down more!!!!
If you can maintain PO4 at .03 on the Hanna meter you will be fine. No reason to go lower.
My readings on my Hanna Phosphate Meter stayed around .11 and .07. I made water changes at 5% tank volume once a week for about two months and it still stayed around .07. When I finally cleaned the inside of the test tube, the reading dropped to .05. So long story short, the test tubes need to be cleaned as well. You'll see a marked improved in LPS corals such as Chalices when the phosphate levels are below .05.