Setting upvc window back from plaster line

[Discoeye]

Decided to start my window install as the weather is on our side at the moment. On taking off the window trim on the inside Ihave found a 10mm gap between window and plaster line and  its an overall 30 mm gap from window to tile line.

On the outside the window is set back 30mm from the render line so my thinking is cut back the plaster line to the tiles so I can set the window up against the tiles, that would make the window around 60mm back from the render. Is this set back to far im not sure if its a cavity wall as the original house is solid 9 inch brickwork and the window is on a flat roof extension at the back of the house and haven't taken the window out yet just the internal trim and found the gaps.