Minimum 50% activity -- this means that IF you are going to be stalking, that at least half of your focus/energy for the round has to be put into the action of stalking. (50% means moving at least 1/4 of your BMR).
I can only give an example of what I would do.
Stalking and Drawing a Dagger -- Personally, I would allow both of those actions with only a minor reduction in distance traveled.
For example (presuming a successful staking roll), if your character had a BMR of 80'/rnd, and you were stalking, you BMR for stalking would be 40'/rnd. In the round in which you drew the dagger, I would normally just say that you only traveled 35' that round, and leave it at that.
The exception would be if there were difficult terrain or other special factors that might cause a stalking failure. In such cases, I might penalize the roll as well when trying to something else because the attention devoted to drawing the weapon silently might be enough to cause a failure in the stalking (i.e. make him stumble or stub his toe and make a noise that alerts those he is attempting to stalk past, etc..)
Very situational, IMO...
What the activity percentage system attempts to do is to give you the tools that you need to be able to better handle such tasks/situations on the fly, and with as little muss and fuss as possible.
