If we consider that (one of the many definitions) of momentum is:
momentum(data,periods) = 100 * data / Ref(data, -periods)
then we can compute the rate of change (in percentage) as
RateOfChangeofMomentum(periods) = 100 * momentum / Ref(momentum, -periods)
If you want you can scale this value too...
myMom := data / Ref(data, -prd1);
rocMom := (100 * myMom / Ref(myMom, -prd2)) - 100;
We can achieve the same results using the built in functions for momentum and ROC:
Hope this helps.
P.S. All this information is easily discovered by reading your MS Users Manual and completing all of the exercises in the free Equis Formula Primer.