I would love to chat with any knowledgeable Gossen LunaPro users
I suggest that you download a manual for both the Luna Pro and the attachment from the Gossen web site.
The underneath of the attachment should have a chart as follows:
15deg = +1 EV
7.5deg= +3 EV
This means you have to add either +1 or +3 EV exposure to your reading. i.e. you have to add that much exposure to your normal reading. If It shows F8 @ 1/250th then you have to use F8 @ 1/125th(+1) or 1/30th(+3) respectively.
This unit is also usable on the Luna Pro SBC.