They are incompetent if they say that. Truest Worral has been re- arching springs for 70+ years. They know what they are doing. Results last as long as the original spring did.
Interesting thread,
My 69 convertible is dragging its backside a bit and would like to have the ride height slightly higher than level. What amount of recurve should I request?
Pardon the intrusion.
You request what you want the end result to be. For example “I want the rear of the car to be XXX inches higher”.
Simple enough.
Cheers Royce
Before you have the height raised, consider carefully the reason. If you wish to put very large tires back there, perhaps it makes sense. But I know someone 
who had them arched an inch higher with 15” wheels for purely aesthetic reasons and discovered there is a large gap now above the rear wheels that looks odd. It handles well but likely not as well as if it were the standard height.