what's a good starting bankroll for one session at a .50 9/6 jacks or better machine
There are several bankroll estimators available online, However, a game MUST be positive (>100%) for them to work. There is no bankroll large enough for a negative payback machine like 9/6 JOB.
However, you asked about a "session" bankroll without defining exactly what a session is for you. So, I will use a method I often use to understand risks.
In JOB the only hands with a long frequency are the royal flush and straight flush. The payback of the game when you eliminate these hands is 97%. Therefore, you can expect to lose around 3% of your coin-in inbetween these hands. For myself, at 800 hands/hour and .50 denom I would be betting $2000/hour. A 3% loss would be $60/hour. Now, compute you session length and you have an good esitmate for your average loss (4 hours -> $240).
Finally, you need more than your average loss to have a high probability of lasting an entire session. Since JOB is a relatively low volatility game I would multiply the number computed above by 2-4 to get a good chance at completing a session. Naturally, the longer the session the lower the multiplier needs to be since you become more likely to compensate for bad streaks.