Hi, good question !
As you are a consultant and not a scientist - you want to make sure that the "final" answer (better: answers) is seen:
a) in the light of the assumptions that you took
b) how a change in those assumptions would affect your answer and
c) how you would go about to sense-check or precise your answer going forward (this builds trust with clients and offers opportunity for next business!).
So how could a final solution look like?
1) There are about 45-55 mn bicycles in Europe (as with recommendations, jump straight into the result, no need to talk about anything else here)
2) ...based on a bottom up (45mn) and a top down appraoch (55mn)
3) ...approach 1 seems more exact due to x, y , z, hence the result is more skewed towards 45mn
4) The answer is most sensitive to assumption x. That means that a change of #% in x would alter the answer by y%
5) ...In order to precise the answers going forward, there are 2 possibitlies:
...precise estimate of x by means of xyz
...conduct a 3rd approach (time consuming) and then "triangulate" about the results.
Hope that helps!