Can anyone please confirm to me the accepted way to calculate strength of schedule as a secondary TIE breaker. I've found varying ways of calculating it but not sure which one is most correct.

The FFG Tournament rules outlines how strength of schedule is calculated:

If players have the same number of tournament points, any player that has played and defeated all other players with the same number of tournament points is ranked above the other players. After checking for head-to-head tiebreakers, the tie is broken based upon Margin of Victory. The player with the highest Margin of Victory wins the tie and is ranked above all other players with the same number of tournament points. If the players are still tied after Margin of Victory, calculate the strength of each player’s schedule by combining total match points of all their opponents. The player whose opponents have the highest total combined match points wins the tie and is ranked above all other players with the same number of tournament points. If players are still tied after strength of schedule, players are sorted randomly.

Tournament Software Strength of Schedule

FFG’s tournament software, TOME, is currently in beta and accessible to some tournament organizers. This software uses a different strength of schedule calculation than what is outlined in this document. We have made this change to improve the tournament experience for players and more accurately represent each player’s actual standing in the tournament. TOME calculates a player’s strength of schedule by dividing each opponent’s score by the number of rounds the opponent has played, then adding the results of all opponents played by that player. The software then divides that total by the number of rounds the player has played to find a player’s strength of schedule. Tournament organizers using TOME should communicate that use to players before the tournament starts so that players are aware of the different strength of schedule.

So you have two ways to go about doing strength of schedule. You can go the TOME route, or you can go the other route outlined above.