As it turns out, adjusting the distance of the installation from the threshold (rather than an adjustment to the approach angle, which was larger than I had considered), to achieve the required MEHT for the runway (presumably for the largest type likely to use it) is quite right. I found the following link, which includes the formula for MEHT v Distance from threshold too, for those interested. I've learnt something new.
https://www.tc.gc.ca/eng/civilaviati...2-009-1202.htm