Figure out what the rationalization of 3 ft. If the 3 ft is measured, use the 3 ft. If it's a wild guess, then offer options.

"I can give you x displacement with the simplest design."

"I can get you this much more if I add the spring."

"I can get you this much more with a loop."

"I can get you this much more with a loop and a spring."

"Do you want me to keep going? Others typically don't see this much displacement."