This is not an easy one to answer as the height of the windscreen determines whether you will need a demisting/defrosting device or not.
The best way of showing the calculations is to quote directly from the VOSA standard IVA guide book.
The windscreen "horizontal plane" is obtained by measuring/projecting a line through a series
of datum points (which are based on the requirements of E( Directive 78/317) from the
driving position to its point of intersection with the windscreen.
The driver's seat, if adjustable, should be adjusted to its rearmost/lowest position.
i. In the case of a seat with an adjustable back rest, the measurements should be made with the seat back rest adjusted to an angle of 25°, line 'A' being parallel to the centre line of the seat back rest.
ii. In the case of a seat with a fixed back rest, the dimensions 'A' and 'B' should be as follows.
The windscreen "horizontal plane" is at the point where the projection of line 'C' intersects the windscreen/windshield, or if above the windscreen/windshield, the vertically projected plane of the windscreen/windshield (line 'D').
Got it! thought so 