Using a historic rule in forecasting

Has anyone any insight into using a historic rule in forecasting? I am currently capturing a couple of key variables in the result pin table then using an historic rule to compare the value this period to last period and if there is a difference recalculate the entitlement.

But this is not working correctly in a forecast and i am wondering whether this is because in forecasting we are not actually writing values to the result table therefore the historic rule returns nothing from the last period.

Is that assumption correct?

Thanks for the input!