The troubles with that approach are that a) we've tried it, and it doesn't work very well, b) it's complex - how much is 'too much', how much do you 'take from

The troubles with that approach are that a) we've tried it, and it doesn't work very well, b) it's complex - how much is 'too much', how much do you 'take from the top', and how much do you 'give to the bottom', and c) it's expensive to administer, because a lot of time it spent working out the numbers and scheduling (and monitoring to prevent fraud) involved in the (b) bit.