Pla, Yes measuring at the widest spot will work fine. For a 48" cub (you never really said which one) Half inch throw on elevator even for high rate is too much and it will be a handful. Do not use more than the plans suggest for either hi or lo rates and if you error do it on the low side for all the surface's.

This is the most common error the newbie makes on arrival at the field And more time is lost for this reason than any other perhaps even being unable to fly on that day. Most people use reasoning that goes something like this: "I want all the control I can get" well this is opposite of what is needed. Yes, on rare occasions there is not enough but this is almost never the case.

Welcome to the adventure.

John