I couldn't find the two-stop recommendation on the Tiffen site within a few minutes looking.

x8 (three stops) used to be printed on the slip of paper you got with your typical filter.
-If you want to keep it simple you can use that factor and be fine.

Adrian mentioned the instructions and chart from Gordon Hutchings.
-In the specific example of the 25 Red filter, AFTER reading through the filter, you open up an ADDITIONAL two stops.

Hutchings method sounds like a good way to go.