location of entry pipe

[jerrywh]

 In Flehto's example the gun has a unusual entry thimble that is extended. There is nothing wrong with that at all but it throws off the visual effect artistically. In such a case the location would have to be shifted in order to maintain artistic balance.  In my opinion that is the discrepancy as it should be.
 Good job. Nice gun Fred.
 

[Mark Elliott]

I use the 3/5 (breech to muzzle) rule to put me in the right neighborhood and then make the final decision on the rear entry (step) by eye.    That said,  I think Jerry's rule also makes good sense.