The "screws" are sheared at factory and also sealed. Most drill them or multigrip them out and replace them with standard ones - usually resulting in a stuffed idle (also check the FIV).
As for using multimeters etc - enjoy. You still rarely get it spot on - if ever. You can turn all you want - but when you tighten the screws it can mess the setting.
Hey - what do I know? Apart from the fact that a TB with sealed TPS off an XSi cost me Au$50 (less than that in US currency) and it bolted straight onto my SiR's B16A and immediately fixed 90% of my idle problem (rest was the FIV). That was after cursing the dickhead in Japan that had messed with the TPS.