Considering there's a top 3 reward, I wouldn't be surprised if they made it based on the last day's rating . I think that way's better honestly, as long as they release percentile marks for reward tiers instead of straight up Elo marks.
Well if you change it so it's based on the last day ELO rating it would encourage people to get their rating and then sit on it. I did it for season 1 because I hate ranked arenas so after I got my 1500 rating I stopped queuing. At least if it is based on highest ELO achieved people can still queue afterwards without fear of losing their rewards.
I'm not sure how the % thing would work or what the % is based off of. I'm also curious what % of people managed a 1500 rating or higher. I never see more than myself and maybe one other person on fleet with a rancor mount.