Converting Tremulus Lune to work with fet instead of optocoupler?

[freddd]

I've looked around and seen various diagrams of how to use a fet as a variable resistor, unfortunatly they never really have any guidelines with regards starting values.
I was just wondering what sort of values I would need to start off converting the trem lune to a jfet resistor instead of the standard opto arrangement.
Any help/pointing in the right direction appreciated
Thanks in advance...