Yes, the adjustment is from 100%. The specific algorithm is explained in this support article:
It’s assumed that your water times you set in programs are the typical water times for an ‘average’ day, defined as 30% average humidity, 70 degree F temperature, and no rain fall. Hence if humidity is 30%, temperature is 70 F, and there is no rain, then it would calculate 0% adjustment, and hence the actual water time is 100% (no change) of your programmed water times. As the weather condition deviates from the baseline, the adjustment will change according to the algorithm. In sum, the formula calculates an adjusted percentage that deviates from 100%.