it must be that
k = 9.
Please let me know if I have messed up something.
I think you're right, but I also seem to remember that the jackpot does indeed pay out an extra 9 times your stake in addition to your regular winnings.
Take this example:

He bet 0.00400303 at 1.035x and won. That should give a profit of around 0.00014011
But he also hit the jackpot and so won an extra 9 times his stake:
>>> (99 / 95.65 - 1) * 0.00400303 + 9 * 0.00400303
0.03616747021432305
If he had lost, his profit would have been 8 times his stake (9 from the jackpot, -1 from losing the bet)