First, a bankroll is not what you bring with you. That could be your trip bank or session bank. Your bankroll is the total amount you have for gambling. But the size of your bets relative to your bank are important only in that you want to minimize your periods of inactivity. There must be some limit on the amount that you are willing to contribute to gambling. Decide what it is going to be over some reasonable period of time, 18 months perhaps. Size your bets to some reasonable risk of ruin, that's up to you, based on the total amount you plan to contribute during that period. All risks of ruin less than about 13.5% are largely a matter of opinion based on personal levels of risk tolerance. Anything larger than this grows your bank more slowly because of excessive volatility, assuming you adjust your bet sizes at some point in the future based on wins or losses.
If you are the type of player who makes some fixed number of trips per year, and generally brings the same amount with you regardless of the wins or losses of the previous trip, then there is no such thing as total risk of ruin. You have an undefined bank. Just decide what kind of trip risk of ruin would make you comfortable and apply that formula.
Since you are talking about "the amount you bring with you" I assume you are the second type of player and don't really have a defined bankroll. The important thing to remember for this type of player is that it is perfectly ok to go broke. If you keep going with the same amount of money and play a positive expectation game, the odds are in your favor to eventually come out far ahead. After losing three or four session banks you should not be hitting your head against the wall and asking people what you are doing wrong. You could well be doing everything right.