There is no precise formula for BR when multitabling.. Basically, its a know fact that your WR in terms of PTBB/100 decreases as you increase the number of tables..Its player dependant. Some can possibly have 6ptbb 2 tabling but have 3ptbb 8 tabling and so on. Only concern is that lower PTBB means higher variance and thus greater BR requirements. A well round, tilt free, A-B game player should suffice with 25 buyins. However, in most cases 40 is a must.. Since once you hit a downswing, expectially at higher stakes, you need to focus to keep playing at least your B game, otherwise you will just amplify your downswing. I recently had a 21 buy in downsing that took short time to recover, but ive noticed that if I played perfectly, it would have been only a max 15 buy in downswing. Long point short, large BR is more intended for psychological support, to give you more comfort, rather than to protect yourself from variance
GL