ᐅ Minimum height for window lintel / insulating window reveals

Hello everyone,

We have just ordered an external roller shutter box for facing brick from Roma. We have dimensions of 230 x 260 at the top and 270 x 290 (W) x (H) at the bottom. Can someone tell me how much of my lintel will be visible below the ring beam, or when standing in front of the window (inside), how much of it should be visible?

Now to my problem:

I informed my bricklayer that our window installer recommended 32cm for the lintel. It has a slight curve, measuring between 30.5–31cm in the middle and 33–34cm at the sides. Could there be an issue in the middle if the lintel is only 30.5cm, since the Roma box is 29cm high? That should be sufficient, right?

The suggested alternative is to have the lintel lowered by one brick course and rebuilt, but that would make it 8cm lower... This was my first concern 🙂

Second question: Is it advisable to insulate the window edges on the outside with Styrodur / Jackodur rigid foam boards (wall structure: 24cm (9.5 inches) pumice, 10cm (4 inches) mineral wool, 11cm (4.3 inches) facing brick)?

Thanks for your help,