FX Impact scope rail height

"Helimech"To get proper eye relief, I'd go with a high mount. Adjustable rings would be the best bet.
I'm accustomed that scope is max 10mm (bit less than 1/2") above the barrel/shroud
On bullpup designs, since cheek piece is in level or higher than barrel, scopes are a lot higher and that amplifies all kinds of canting errors.

I had a scope rail on Steyr LG110 so gap between scope and barrel was 2,5cm (1") and when I've removed it, groups on 50m (55y) reduced in size

With this design, and by sniperlabyo's measurements, 1 1/4" to barrel then additional 1" of mounts comes close 5cm (2") between scope and barrel (not scope center to bore, just scope outer ring to barrel, scope center to bore is about 8cm (3"+))

In my experience, low mounts would reduce canting errors and look more streamline