Do you mean 'Where is the right place to locate the start for a timed mountain climb?' or in effect how do you decide where the mountain has ended and the foothills begun?
If someone was trying to cycle over a mountain pass (such as Hardknott Pass or Kirkstone Pass in the Lake District in England) within a given time, or trying to beat another's time, then deciding exactly where the start and finish points were could make a big difference. In the case of Kirkstone Pass you might take the points where the gradient first becomes within, say, 2% of level at either side, or from the first houses of the villages at either side of the pass, or even the shores of the lakes in the two valleys connected by the pass.
Can be open to debate - there may be a point on the road where there is a marked increase in gradient, or sometimes the point where cultivated land ends might be taken as the beginning of the mountain. The question of equal difficulty in either direction might arise. E.g. if the course was between the shore of Ullswater and the shore of Windermere over Kirkstone Pass, there is the point that as Windermere is 39m and Ullswater 145m above sea level, going one way involves climbing 106m more than the other, and so a record for going the 'downhill' way might be seen as a lesser achievement.