From what I understand, you should increase your BR requirements inversely.
For example, the general consensus for BR is to have 20 buy-ins. Let's say your normal win rate is 6BB/hr when you're single tabling. When you switch to Multitabling you now run at 4BB/hr and are now 4/6ths or 2/3rds of your normal win rate. Now, apply that inversely to your 20 buy-in rule..
3/2 x 20 = 30 buy ins.
As far as finding your actual win rate, here is an excellent post that talks about this.
http://forumserver.twoplustwo.com/showfl...=8#Post12784921