To obtain the result, you need four things:
1) BetID: You can obtain this from your "My Bets" column.
2) No. of challenges: Number of challenges issued to opponent.
3) Total Bets: This is the total PVP games played by user, this can be obtained from the "Profile Settings" page on the top left corner of the page. Please note this must be your total bets at the time of the game; not to be confused with your current Total Bets.
4) SEED: This is automatically generated by the server, although you may change it whenever you like; found on the top left corner of the page next to the logout button.
Steps to obtain a user's roll number:
1) Combine (1#2#3#4) in sequence.
I don't understand your provably fair method at all. It seems the 4 variables to calculate the roll outcome are known to the player in advance. This would mean the player can generate their own results in advance and therefor cheat.