How to Adjust Factory Tachometer?

[badpeanut]

I picked up a factory tach from a big truck and just finished installing it and a reproduction vacuum gauge in my Blazer. Unfortunately the tach feels like it reads a few hundred RPM high. I have HEI and a late 1970's Camaro tach filter. I'm going to temporarily hang an aftermarket tach under the dash for comparison. I read in other posts about the calibration screw on the backside of the factory tachs:

1) Can I reach this screw without removing the gauge cluster (is the circuit board in the way, do I need Ethiopian arms, etc)?

2) Is this screw a flat head or Philips head?

The tach currently sits nicely at zero RPM with the ignition off.