Create a data validation say in K4 having the different month names. Use this in J5 and drag down

=IFERROR(IF(1+$K$2<G5/INDEX($D5:$G5,MATCH($K$4,$D$4:$G$4,0)),(G5/INDEX($D5:$G5,MATCH($K$4,$D$4:$G$4,0)))-1,"OK"),"OK")