I think they want you to take off the amount of rib surface that will EQUAL the "thickness" of the sheeting you want to apply. I am assuming that the sheeting is to be 1/32" thick balsa. Since it is a glider, I guess they are trying to minimize the added weight. I was building a 4* 120 and was going to sheet the wing in the same manner. That sheeting was 3/32" thick.
I ended up passing on the modification. A lot of guys have done this succesfully. (I guess this goes without saying.... it is a LOT easier to do the rib mod before assembly of the wing)!!...lownslo

Bob