A Little Confused About Dex per w.atk and how it differs from STR-less to low STR

So more like

dex/atk = [1.8*(1+0.9*mastery)*dex + str]/[1.8*(1+0.9*mastery)*atk]
for an exact value

That comes with the same results as my formula, somehow.
dex/atk = (dex+(1/((3.6+3.6*0.9*mastery)/2))*str)/atk

Yours looks nicer though. No idea where you got that 1.8*.... oh now I get it. Good thinking. It's your own little way of averaging.