ok this has always bothering me and I guess it's been long enough to ask the question.
So 3d sketch dimensioning:
The only way to put that 50 dimension is to click LMB somewhere outside that isn't part of existing model - if not that it will pick it for selection of dimension.
Is there a way to click hold something (like ALT in so it skips automatic snap).
This is only a case when adding dimension for single line LENGTH.