if you are looking for hints on how to get the best out of planetary focusing, try the steps recommended in this post : viewtopic.php?t=8409 - those were based on my experience re-testing repeatedly with the same focus scan data with different settings to find out what gave the best detection of the focus point.
Strictly speaking ASCOM focuser drivers are supposed to have non-negative focuser position values - it's all there in the ASCOM documentation stating that the position should be between zero and the maximum inclusive. ZWO aren't the only ones who have missed this
cheers,
Robin