This debate has been ongoing(the issue of orbs etc).Conventional astrology suggests that 10 degrees (conjunctions are permitted) , others say 8, some 6 etc. It gives me a headache just thinking about it.
To be honest with you, in my experience a transit is often deeply internalized AFTER it has gone exact.
At the moment that it is happening(exact), i am not objective to its influence and responding from an instinctive level.
But post its occurance, i review "what just happened?", feel the physical effects(if any) and deal with the aftermath.
It could be a form of "aftershock" i.e when in a life threatening situation, you only realize how close you were to death AFTER its happened etc. Then you have post trauma stress and all that.
So i would say that when still coming towards your 1st house from the 12th, you are still hearing backgound music and perhaps even pyschologically experiencing it. Its not tangible yet but you feel something is about to occur.
But when it is exact, you will no longer be "thinking" of it, but experiencing it.
Then when it has passed that mark, you talk about and possibly re-live, review what had occurred.
So its experienced in 3 stages. Future(approaching), present(exact) and past(separating).
For me, its when it is exact and separating that it is potent. But this is just me. As i like to review and go over things that have already occured in my mind and try to make sense of them.