[RichLockyer]

Their conversion factor varies slightly depending on how much correction is needed.
At 0% correction, the SH is set to 0%
At 11.1% error the SH calibration is set to 10%.
At 10% error the SH calibration is set to 9.1%.
At 9% error the SH calibration is set to 8.3%.
At 8% error the SH calibration is set to 7.4%.
At 7% error the SH calibration is set to 6.5%.
At 6% error the SH calibration is set to 5.7%.

So you have 11.8 in there now... meaning that you assumed a factory error of 13.4%, which indeed would be higher than anyone has ever reported.
If you're 3.3 off now, then it appears that you were indeed at about 10% factory, and you should be able to reprogram the SH for 9.1% and be pretty close.

Otherwise, reset the SH to bypass mode and recheck the speedo.