Sorry, I am late to the party here!
As a general practice I would recommend making size adjustments based on the fit of the Innerwear you have on hand. We know precisely how much each size up or down in length or girth is, so this is more accurate than remeasuring which is bound to be influenced by the interpretation of method and technique when following the written instructions.
The first question I have when a user reports slippage back inside the sleeve is whether the Innerwear has been set-up properly. Often is the case, guys will get the sleeve inside out. This is typically only possible when the baseband has not been folded twice due to the logo being obviously amiss when rightsize out, if not folded. Whereas, inside out with no folds the logo seems to be ok as it is still visible, albeit… upside down.
The problem with having no folds of the baseband and sleeve section turned inside out, one… the overall length becomes longer which contributes to slippage, particularly when sitting, but more critically, the silicone ring is facing outwards which ends up rolling back under itself as the glans easily slides past to retract back inside the sleeve.
This would be the first problem to eliminate before moving into the specifics of sizing and potentially a custom tapered sleeve.
We have a demo video that clarifies the correct set-up, however, it must be sent as a link to your email via DropBox as it has been banned from YouTube. You can shoot me a message at the support email email@example.com if you need the link.
What I can offer here is a couple of pics to show the difference between inside out with no folds…
and right side out with the proper folds…
Hope that clarifies the first step in troubleshooting an improper fit.
Let me know how you make out once you have clarified this aspect.