For me personally BRM are a very personal matter and depends/varies from player to player, the factors that affect it are mostly...
* Your "TRUE" ROI/Winrate. This is the ultimate variance killer, higher ROI/Winrates will make you less sensitive to downswings meaning they wont come often and they wont last long. Lower ROI/Winrates players will also need more buy ins to play certain formats than those with higher ones.
* Monthly volume. The more you play, the faster you will get out of downers and/or the more you will maximize your hourly. Low volume players will need a lot more buyins toplay certain formats than those that do play a solid/standard number of games/
hands a month.
* Prone to tilt. This is very important, usually the less prone to tilt you are towards variance the more capable you will be to sustain you´re A-Game the longest on your session maximizing the EV of your decissions. Very sensitive guys, those that get upset everytime their AA gets cracked by 72, will see their quality of decission lowered dramatically, hence will be needing more buy ins than those that can unplugged their feelings/emotions from their short term results.
* Posbility to reloads. If you have the chance to reload your bankroll then you wont be needing a much buy ins as someone with less chances. Guys with posbility to reloads will need less buy ins on their cashier than someone that doesnt.
* How often you study. This will help you keep your thought process/Decission making sharp and ahead of the learning curve of poker. Those that reraly study will need more buy ins on their cashier than someone that is constantly posting hands on forums, analyzing hands on study groups or just with other more experience players, etc.
Once you get all this factors just play around with a variance calculator to the statisticall likelyhood of the worst posible scenarios, i use this one...
http://pokerdope.com/
Hope this helps, cheers.