Choochoo999: You need to have an average of 58 or better and 500 games after they introduce the Champion Tier.
Sorry, both is definitively wrong.
Some TLs tiered up with averages above your number, so 59.000 is the most likely threshold. 200 "extra" rounds have been explicitly named by Icon.
59.000 is also quite logical, compared to the 61.00 => Legend and 60.000 => TL.
The complete rounds number necessary for new TLs hasn't been discussed much, but reliable sources stated "200" as well.
Common conception appears to be max. 59.000 average plus 200 valid rounds from the introduction, whether players were TLs then or not.
A "special gift": Saturated TLs were robbed of their 300 best rounds when C and TC became public, causing their averages to jump up. Unsaturated TLs: dunno.