I am looking to fit a Marble universal rear sight and a contour front sight to my Interarms X (7mm RM); and I notice that they do them in a variety of heights.

How do I determine the correct height? I presume if its badly wrong I won't be able to adjust for the whole trajectory of the round, but I have no idea how to work it out!

To be able to have the widest latitude of adjustment, I would try to find a combination of front and rear sight that would put your line of sight as close to being converging with the line of the center of the bore at 100 yards as you can. You can do this by boresighting at 100 yards, or use an optical collimator inserted into the bore.

Then, after firng a group at a measured distance, you can use this formula to make further sight adjustments, if necessary:

The formula used to determine the AMOUNT of iron sight change needed is: M = S * D / (R * 12)

Where:
M = amount of movement or change in sight height needed (in inches)
D = distance you want to move the bullet's strike
R = range to target in FEET
S = distance between the front edge of the rear sight and the rear edge of the front sight blade (in inches) (sight radius)

As an example, assume your sight radius is 5", and you need to move the bullet's strike 3" higher at 25 yds (75 feet):

M = 5 * 3 / (75 *12) = .016
thus you need to lower the front sight (or raise the rear sight) by .016 inches.

Raise the rear sight or lower the front to raise the point of impact, and vice-versa.

I use this:

Sight Hieght Calculator at Brownells