We reach level 1
Jackpot > 10 000 tron - On the road for level 2 Interesting concept.
What if I’m the first and only to join the game. Do I then lose 20% of what I put in and win the other 80%?
No if you are the only player to play you also win the totality of the 20%.
The 20% are dived between all existing shares. Winner's share(s) include. The contract don't verify if winner already get shares inside to exclude this address from the split. He just split between all existing shares.
So if you buy for example the 10 first shares from level 0 and nobody play after you, once the jackpot get hit by you the contract will dived the 20% between your 10 shares.
Another example, if you bought 1000 shares at level 1 and game goes on and let's imagine you buy 1 share during level 6 and win the jackpot with it, once the contract split the 20% for all players you you get your part of it calculate on the 1001 share you own, winner share include.