Your question is confusing. What type of "compression" fitting are you alluding to? There are various types and makes. Is this for plastic, copper, or SS materials?
One conventional type of compression fitting uses a ferrule that is slipped over a copper tube and which it "compresses" into a hermetic seal once a back-up nut is tightened over the corresponding receptive connection. Is this what you are referring to? If you can't describe your fitting then give a make or manufacturer and its model or type. What specifically do you mean by "where does a tube end"? Are you referring to the location where the tube end stops or terminates within the fitting?
Tube manufacturers like Swagelok or Parker will have all kinds of information on this topic, along with the dimensions. If all else fails, just get a piece of tubing, and a fitting, and try it. You'll feel the tubing hitting the shoulder of the fitting when it's pushed home. Mark the tubing at the nut, pull it out, and measure it.
Wes,from a maintenance standpoint I never bottom out the tube in the fitting. Whenever you break this connection and have to replace a valve,etc,if the tube has been shoved in till it bottoms out in fitting ,it can hinder you getting the compression nut started again. Maytag
