In Team Fortress 2 I turned on sv_cheats and kept lowering the fov and then comparing it to the zoom fov. I reckon you could do the same thing in other Source games. The Team Fortress 2 sniper zoom fov is 20 so I guess you're right.
Ah, I was actually wondering what the TF2 sniper fov was as well, and guessed it was probably around 20. Thank you for confirming it. Then there is the matter of zoom sensitivity ratios I suppose - it seems about the same in both games on default.
Dafuq do you want it for?! Quake istn't CS and even if you make your HUD (by zoomhud you probably mean scope-effect from awp?) look like it it still won't turn into CS. Movement is different, weapons are different, models are different, architecture is different, goals are different.
It's not a concern for me if he wants a CS scope-zoom just for aim reasons. But the way he states it, he wants Quake to look and feel like CS and that won't be possible even with a zoom-hud.