rans
glad you corrected yourself
it really isn't that hard...always add five knots to vref
do what you said up to twenty knots
if you are in a situation where the numbers work out to adding more than twenty knots, consider going to another airport.
HOWEVER
the DC9 is quite able to tolerate higher speeds...I've seen vref plus 50 with flaps 50 (which are no longer allowed for noise abate) but the runway was 10,000 long and the winds were a bitch plus.
normally touchdown at vref to vref minus five.
I encourage you to read a book called, "fly the wing" by webb...a former DC9 checkairman for eastern and an acquaintance (RIP).
he has a great deal to say about wings and wind...and even use of a higher Vr for rotation on a wind shear kind of day. Fascinating stuff for any pilot.
DP Davies book is fine too.
I know one TWA pilot who was called into the office for landing a 767 at KPHL with vref plus 25...he was just told to divert next time.
so its up to you...if you have to add more than 20 knots, divert or be a test pilot