@jdanielp: I never did measure the previous one to make a direct comparison, but I would assume it's slightly shorter.
But the fashion is for MTB frames to have rather slacker geometry these days, so you might find the that two factors cancel out.
I'd have thought that there is so much potential variation in frame geometries between different bikes that it's pretty difficult to arrive at a reliable answer to steveo's question. But urchaidh's approach is the most likely to work for a given bike: wheelbase (i.e. the distance between the front and rear contact patches, often quoted by manufacturers) plus the diameter of one wheel including the width of the tyre.