I haven't updated my installation yet, but if I remember correctly, in Diabotical's settings, you could specify which game your FOV value came from.

I think the default FOV mode is vertical FOV.

A vertical FOV of 86.6 should be equivalent to a QL FOV of 103.

EDIT: Okay, my update is complete, and I can confirm the conversion is available in the settings.

It's at Settings -> Weapons -> FIELD OF VIEW -> FOV Converter.
Selecting "Quake/Source" for FOV Measurement and entering QL FOV values should do the trick.

using their quake settings for the FOV feels completely normal to me