Index Match (wrong result)

Try

=INDEX($D$1:$KQ$1,MATCH("YES",(INDEX(D2:KQ2,MATCH(A1,$D$1:$KQ$1,0)):INDEX(D2:KQ2,MATCH(B1,$D$1:$KQ$1,0))),0)+(300-$A$1))

Your highlighted MATCH returns the position relative to the sub-set created by the INDEX statements (43 in this case) so INDEX($D$1:$KQ$1 will find the 43rd value from D1 i.e 258. You need to add the OFFSET (300-A1) to get position relative to D1.

Make sense?