How far back are you looking at the results? How much weight are you giving higher prize money tournaments? Are you factoring in lan results? and so forth.
For this single event, I'll be basing it on tournament prize value. So player A has won $3200 worth of events, and player B has won $1800 worth, blah blah blah. It'll be fairly estimated for this event only. As I said in the OP, I'll be introducing a points system to manage future seeding. I'm honestly only doing this to encourage more people to play, as it's demoralising to open up the bracket and see yourself against X, Y or Z in round 1. I've already started compiling a spreadsheet of tournament results and average value, so I'm not phased by the task.
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