It would help me to do the calculations if you measured the voltages at the base of Q1. when the screen is at 500 +2.609v and again when it is at 290 +4.06v and 999 +1.578v.
Also, when the pot is at the centre, disconnect the computer and measure the current coming out of Q2 ie. connect your multimeter between the collector and 0V. 61.4 µA
Then re-connect the computer, set the screen to 290, then disconnect and measure the current again107.0 µA. repeat for 999.I can't easily take this measurement because there's a spring on the pot and I can't hold it at 999 and then disconnect it from the comptuer without it moving. However, pushing it as far left as it will go, I get 8.3 µA. This is not the same spot that I measured 1.578v above, though. When I measured that, it was easy to hold it at 999 and look at the volt meter. If it's really critical that I get that measurement, I'll rig something up to make it easier.