I have an Auber temp control probe mounted a couple of inches above yours and the probe is about +30f then the dome lid thermo on my 18.5 WSM.
Jim - KCBS CBJ
Porcine Perfection BBQ Team
18.5" WSM | Packer Green Thermapen
| Auber PID Temperature Controller | Mini WSM | Green Bay Packer UDS
GO PACK GO!