Hi @dmandy, thanks for the confirmation. To implement a custom calibration for your project (I assume you are using a Tobii 4C Eye Tracker?) then a couple of routes are open to you but unfortunately, no custom calibration routines are available directly with the Core SDK.
Firstly, you can indeed use the Tobii Pro SDK in conjunction with the 4C but this requires the purchase of a special unlock file to do so however this is likely to be the simplest route to implement (some examples are included) and integrate into an existing application assuming you can port your existing Core SDK code into Pro SDK within .NET without too much issue.
As a second option, custom calibrations are available via the Tobii Stream Engine which is written in c++. Info regarding this SDK can be found @ https://developer.tobii.com/consumer-eye-trackers/stream-engine/getting-started/
Again, this requires the purchase of a special licence to use for custom calibrations. This would be a better option if you are happy to create an application from the ground up using C++ and to target multiple platforms.
Please let us know how you would like to proceed and if we can answer any further questions you may have.