Doubling up and installing windows

Hello,
I had dormer windows installed and now the windows still need to be put in. I have an idea of how it should work, but I'd rather ask again :-)
The beams are 12cm. I need 16cm for the insulation (mineral wool + vapor barrier foil), so I wanted to screw on 48x48mm battens everywhere (makes 168mm). The window (Energeto 8000, triple glazed) will be wrapped with the tapes as usual, aligned, screwed in, foamed, and the adhesive tapes folded over (outside under white side panels (pictures), inside on vapor barrier foil). The metal window sill lies at the bottom in the first window groove? From the space (2cm to white panels + 48mm battens + 24mm intermediate layer for sockets + drywall), the window fits in without an inside window sill. Maybe a 5cm window sill that looks into the room? If I now have 140x140cm from wood to wood, do I then order 138x138cm? At that size probably two-leafed?
Many thanks!

Unfortunately cannot be edited:
Installation level = 48mm = 9-10cm interior windowsill, if it protrudes 2cm into the room
Window has 85mm.
How do I seal with external window tape? Stick on external adhesive tape and hide it with a decorative strip?
Installation as usual means according to RAL.
If I now place the 48x48mm battens 1cm away from the window, I could buy windows that fit directly flush against the white panels, since there is a 1cm foam gap at the battens.