@BigBen2k.
With that system you could connect directly to a variable resistance (turning potentiometer) and get a linear output without any complicated schematic , just the variable resistance and resistive divisor connected to the 12v. Cant get any simpler than that . I've got a similar one in a process simulator . I'll get some pictures tomorrow if i have the time.
@Acturius
Something like this? :
http://clientes.netvisao.pt/carlo001/06.jpg