mjtibbs
The formula for the glideslope is only a guide, albiet a good one, as to what you will need to descend at on the approach in order to keep the glideslope needle centered. Just half your groundspeed and add a zero. ie. 160 kts = 80 plus 0 = 800.......
That should get you there, and if all else fails, set 750fpm ROD and adjust as neccesary.
You cannot change the Glideslope descent rate from the flight deck as it is a fixed transmitter on the ground, usually set at about 3.3 degrees. I am certain you were flying a backcourse approach as well, judging by what you said the needles were doing.
Happy flying