Hi @stumblingdrunk, thanks for your query and sorry to hear about your issues. Certainly, if you are not using the mounting bracket as supplied with your Tobii 4C eye tracker then unfortunately some degree of inaccuracy is to be expected and officially we cannot support any other mounting systems.
However, what might be of use to you is to check out this other forum thread @ https://developer.tobii.com/community/forums/topic/ultrawide-monitors/ wherein a number of users with ultrawide monitors have posted their solutions that seem to work well, so hopefully you should be able to reproduce their setup.
It may also be helpful to improve calibration accuracy by navigating to the Tobii Eye Tracking icon in the system tray, select your name then select the “test and improve calibration” option which may reduce issues along the top of your screen.
Accordingly, please go through the steps I have outlined and kindly let us know how you get on.