Basically, I take the top 7 rated songs of each year for the decade per Billboard. First everyone ranks the #7 songs, which weeds out 6 songs so we are left with a total of 64 songs for the decade (top 7 would mean 70 songs, which we don’t need). Then I try to arrange those 64 into a kind of play-off system, which can be difficult, but mostly it is based on the ranks and the years of the songs.
In the past, this was all Billboard rankings. It should still be BB although they had slightly different methods for ranking early songs.
Too bad. A lot of outstanding tunes either never made or barely cracked the charts.
Dont forget the payola scandals that happened during that time. DJs were paid to promote or play certain tunes so the charts are tainted.