By fixed do you mean adjustable ? I'll probably give it a try.
Meanwhile, direct MB fan header to t-balancer analogue interface :) Not perfect, not quite linear and probably only working through pure chance...
http://www.i2net.me.uk/files/WaterCo...Interface1.jpg
.. good enough for now and not bad for someone with only cookbook electronics skills :D Testing nervously and waiting for it all to go bang :caution: