I had thought the jackpot was just 3/5 * the amount of money you personally had spent playing slots since the last jackpot, but at some point during a long session, it reset on me. It was way past 100k, and now it's about 40k. I still don't have the achievement for winning a 100k jackpot, so did it just disappear?

the jackpot does get increased by 3/5 of the amount spent - but from everyone, not just you. So you can actually track how many are playing at any given moment by watching the jackpot increase.

If anyone wins the jackpot, its then reset to 0 for everyone and needs to start building again.

The best way to earn that achievement is to keep an eye on the jackpot and only play when it hits the high 90s or after its hit 100K.

Ah, so it IS all one pot! Thanks for the clarification.