The absolute risk difference, ARD, is given by:
ARD = R1 - R0
The relative risk difference, RRD, is give by:
RRD = (R1-R0)/R0
So, the ARD is the product of the RRD and R0:
ARD = RRD*R0
In a population with a different baseline level of risk, R0', the RRD is likely to be preserved, so
ARD' = RRD*R0'