It has been awhile since I replaced my TPS but I think I remember in order to get an accurate reading on its initial position, the TVA has to be removed so the throttle body is at dead stops making sure not to mess with any of the adjustment screws on the TB.
I know Sailor put together an excellent breakdown of the TPS but I don't have a copy on the computer I am using to check if the adjustment procedure with the GS911 was included in that. Been awhile since i looked at it. I did mine with the TB removed as I was replacing the TPS anyways so i had access to the TPS screws which are impossible to reach with the TB installed. The TPS can be checked while installed though. Just make sure the TVA is not in place. I left it connected just removed so not attempting to move the TB from base stops.
My PDF document (see link below) was mainly designed for those who (1) either do not have a GS911 -OR- (2) need to do more advanced troubleshooting if TPS is intermittent or defective -OR- (3) have messed-up with "tamper-proof" screws with blue paint at wrong time or wrong setting.
Of course the GS911 has an interface to check the "basic" TPS setting at idle stop, HOWEVER the procedure is a bit confusing when you follow their screen prompts. Last year, I did a bit it more "static" tests in garage using GS911 software (version 1411.2 not-beta) and using the VoltMeter method to compare. Found a few useful items like:
(1) Their TPS setting / adjusting interface is bit better than 2 years ago. The only part that is still confusing for novice if the 1st or 2nd screen where you see you are above green and they tell you to move steering to check enough free play in cable - TPS should not increase - that is the main message here.
This part can just be skipped and omitted and you go to next screen where eventually the TVA will be either: set to lowest position by GS911 , or they tell you later to unbolt it temporarily.
(2) If the TVA is pulled out from bracket at wrong time, but still plugged in its connector, the piston goes to end and keeps expecting a higher TPS, but it never happens. This is where an error will be logged in engine faults (description of error is not very clear however). If you remove TVA from bracket ONLY at the requested steps by GS911 then you do not get this error. I suspect they reset something internally OR they are able to set Motronic in a special mode.
(3) If indeed you need to change the TPS position, you have to be VERY CAREFULL as 1 torx screw is hidden and can hardly be reached with air-box and Throttle-bodies assy on engine. Also, this must be done with a lot of care as setting will be wrong if done with TVA still in place -OR- if the throttle do not sit at physical idle stop.
You would also need to clear Motronic memory (15 minutes fuse removal with ignition OFF) before you do the 1st engine run after a change. This will also erase any faults from Motronic memory so you can start fresh for your 1st engine run.