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The first 62.5% of people signed up got a guaranteed spot, the remaining 37.5% was randomised from the other sign-ups, that's probably why you don't have a pairing. To be a substitute just post something like "in as sub".
Hey man, I'm GMT +10. I'd really prefer to try and knock it out on the weekend because I'm pretty busy during the week. Do you have a preferred day or time for the weekend?