choosing proximity sensors?

[magicniner]

Anything with a high point that a proximity switch can sense at a repeatable point in the homing direction will do, you could use a single bladed chopper too, anything which sticks out significantly more at one point in it's rotation will do.
Mount the cam near one end of the screw and mount the proximity switch so that it is triggered by the cam.
Simples! ;-)

Here's a pic I found, imagine a gear with just one tooth.