Sorry to post up such a stupid question, but I'm running on caffeine and really struggling to find a quick way to do a really obvious maths question.
Interpolation between two entries in a table:
2250m 2500m
======|======
69.15 | 72.45
Obviously the mean of the 2250 metres and 2500 metres value (ie 2375 metres) is 70.8. How the devil do I find a simple way of getting the value for, say, 2350 metres.
I have a physics degree and my brain seems to have shut down most of the important bits tonight.

