[dickkemp]

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that makes around 15 "buy ins" so around 7.5% of my BR.

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change this figure to ROI. is that 15 BI profit or return. also how many games have you played and what are your intentions with your new found awesome ROI. (which if it is 15 BI return per 10 BIs that is a 50% ROI)
if you are playing for fun 30-50 BIs but you need to move down when you move below 30-50 BIs for that level.
if you are playing pro dont yet. get a job immediately. but if you did go pro 100-200 BIs is ideal. obviously the more BIs the merrier. 1000 game downswings are not unusual same with upswings. another reason why the number of games you have played is so important.
as for the playing multiple tables you should stick to what feels comfortable. as many tables with maintaining the ROI to make it more profitable.
example: 4 tables at a 50% ROI will be exactly as profitable as 8 tables at a 25% ROI. so if you can play 6 tables with a 35% ROI or 8 tables with an 35% ROI that would be maximizing your win rate per hour.
percentage of your bankroll is not important as the total amount in action since they are each individual tables. 50 BIs is still 50 BIs. even if you have 10 open.