You are right that accuracy is questionable. A better way would be to measure on the ground with a spring scale the actual force applied by the spring assist. Then knowing this, go fly and use that same spring scale to apply the forward stick pressure. Thinking thru the process of what is occurring with the spring assist, I think it is logical that the spring tends to add drag in a trimmed cruise configuration. The amount is the only question in my mind. Are we in agreement with that?