Reciprocity Failure and Development

Now that I learned first hand about B&W film's reciprocity failures. I used FP4 and TriX. Ilford films are supp. to be - % development too. So, if you need to subtract 10% in time, as well as say a 10% N-1 dev. time, do you add the two together for minus 20%? Funny thing about this though, I found the sugg. to decr. time on Ilfords film in Steve Simmons book, but Ilfords official website on the films only recommended decr. dev. time. Not sure why.

Thanks again for the help here...and sorry for all the questions recently!

Chris