Bridgeport Spindle Config

I have a Bridgeport Boss 5 mill that I converted to LCNC last year and have it running the spindle with a VFD controlled by the analog out of a Mesa 7i76 board. I have the vfd working to control the speed and direction. I am in the process of removing the varidrive and replacing it with a fixed pulley set with a 2:1 ratio. So the machine will use the vfd to run 500-3500 rpm in high range and 5-500 rpm in low range using the back gear. I want to add an encoder to get tapping to work on the mill and need a little help sorting out how to set up my hal file to allow the encoder to work in both high and low gear. Because of where the encoder is located it will be directly reading the spindle in high gear but not in low gear due to the back gear. I need to set up the hal to have two different pulses per rev, two different volts per rev and a direction switch between low and high gear. I can't be the first person to do this and am hoping someone can point me to a wiki page or a thread with an example I can work form to get this going. I have searched but have not quite found what I need yet.

That looks like a good way to go for the encoder, I still will need to deal with the direction change and the volts/rpm in the two ranges any idea's on that?