Momentum % is quite similar to Momentum. The difference lies in that rather than being the difference between today's price and the one of n days prior, Momentum % is a measure of the day as a percentage of the maximum Momentum.

So:

if P_t today's price and P_t-n the price n days ago,

Momentum is calculated as:

MO_t = P_t - P_t-n

So if you define MO_max the highest absolute value of the momentum, the Momentum % is:

MO_%t = MO_t / MO_max

The interpretation for Momentum % is the same as that for Momentum, purchase when Momentum % changes from negative positive and sell when it is the reverse.