You did the figures on this wrong in a couple of different ways. The first is you essentially included the $10,000 bankroll that you started with into the figures instead of just using the money that was won or lost in the matches. There is an extra $10,000 in your figures that should not be there and that is the $10,000 bank roll you started with. The other thing is you calculated this as if you were not settling up with either your opponent or your player after each of the gambling sessions, but instead played all ten sessions before you settled up with anybody and you then settled them all up at once. This makes a difference because it is then really just one gambling session (and you calculated it as being just one session) instead of ten, and I am sure you meant for it to be ten and not one.
If you did ten separate gambling sessions for $1,000 each, settling up with your opponent and player after each one, then you lost $3,000 for the three sessions that you lost. And your side is $7,000 winner for the seven sessions you won, but that $7,000 was split up between you and your player. Whatever your cut of the $7,000 is, you then have to subtract your $3,000 in loses from that since you take all the losses, and now you have the final figure of what you actually netted or made after all was said and done. And of course there is nothing for the player to subtract out of his cut of the $7,000 since he doesn't take any losses.
So in this case doing an 80/20 split, your cut would have been $800 for each of the seven wins, for a total of $5,600 (7 x $800), minus the $3,000 in losses that you incurred, for a final net total for you of $2,600. Your player in this case would have received $200 for each of the seven wins, for a total of $1,400 (7 x $200), and he doesn't have losses to subtract.