Generally I find that more often than not (now there's a confident statement) that the last bar modified becomes the one primarily affected by subsequent changes. For example, I have alteration up. I then add conjuration. I then add thaumaturgy. If I then go and tinker with conjuration or alteration, thaumaturgy will be primarily affected since that was the one I just did.
Just try three or four different paths at the same time and there will be no rhyme nor reason as to how the allocation goes.
True enough, all subtractions or additions will be done from or to the primary path, but everything else is affected more or less randomly, which is why it'd be important to have a locking mechanism.
Or how about a way to just manually assign points? "I've got a hundred and twenty-three points. Lets put 50 into Construction, 60 into Blood, and 13 into Enchantment." That'd be nice.
Perhaps the current system is meant to demonstrate the difficulties inherent in magical multi-tasking?
__________________
"I think that this situation absolutely requires a really futile and stupid gesture be done on somebody's part"