re: Poker & Percentage to withdraw
I think to answer your question, you shouldn't withdraw any amount of your br unless you are at the desired level that you wish to remain at for an extended period of time (exception being any unexpected life events) ..... you feel perhaps you have plateaued in your growth and need to take time to study some in depth concepts or get coaching etc well at this time you could be making withdrawel's.
Withdrawing will essentially be based on the BRM you are looking to follow, if for example you are playing cash games and follow a 30bi BRM and decide that you will remain at your current level for a while (basically meaning you don't need to build your bankroll to be rolled for the next level up) well then you could at a specific interval say once a month or every couple of months withdraw funds in excess of 30-35 buyins for the current level you are grinding (so if you grind 100nl for example and you have a 5k bankroll at the end of the month then you could withdraw 1500) .... Just an approach I think I would use if I ever got in the situation.
In regards to another way of looking at it you could base your withdrawels on a % of profits that y ou make over a period of time, which would allow you to make withdrawels while still building your bankroll. Say the 20% you mention above and using the same example I indicated about grinding 100nl and you have 5k in the br at the end of the month so 1.5k profits then you would withdraw 300$ which is 20% of the profits for that month which adds an additional 1.2k to y our starting br for the next month and allows you to build your br for higher levels.
Just some of my thoughts on the matter, but I mean I doubt many of use are at the levels we desire to be at so we wouldn't really be following any withdrawel structure. Now if you're talking mtt's then its a different ballgame as basically if you hit a big score you would probably withdraw some of it so that you don't feel hugely overrolled for the limits your playing when you don't feel you are going to moveup your buyin. it's more difficult to have a schedule with mtt's because unlike cash where you would tend to have a steady income mtt's can be so sporatic that some months you may only have losses.