Because you just recently tiered up to Legend, it's actually fairly straightforward to figure out how many ranked rounds you've played at Legend tier.
To me, it appears the rounds from 2/20 on are the Legend-tier rounds. Throwing out the one custom course round, that leaves 19 rounds played at Legend tier. Just including those rounds results in an average 30.89474. Doubling that gives 61.7895 or 61.79, which is your current average.
To keep track, just copy and paste rounds from your score history into an Excel spreadsheet.
Just remember that Par 3s, Par 5s, and custom courses don't count toward calculating average or toward number of ranked rounds played at tier.
I hope this helps!