If you know the distance the hallsensor is switching befor the magnet, there is no problem to use a hallsensor.
Calculate this time within the Ignition Pulse Width time and you have the right time for ignition.
(BTW, this is not correct because the speed of rotation change the magnaeticfeeld and time)
BUT, if you get backslash, the prop is pending between compression, the CDI is firing even if it's turn the wrong way.
When you turn the magnet 90 degrees (left picture), the problem is solved and even with backslash the CDI isn't firing.
Opto's can be durty from pertol or by landing into a plane and also on boats and cars you can get trouble with them.