Yup - you have the reference point in the right spot (edge of the frame) - but I'd do it as a two-component Psi-Value and not use the U-value of just the panel, rather use the Heat-Flow of the entire window assembly (panel+frame) in order to calc your Psi-Install. So something like:
In Flixo, when calc'ing Psi-Values you can tell it to use either the U-value of the element, or use a custom HF for the reference value:
and then pull HF that from a model of the frame+glass. I think the new version even has a 'link' tool so it'll update as the model changes (pretty sure).
hope that helps!