Measuring for cage length

About a month ago, I bought Tickleberry's Spiral which is actually the Twister 2 made by Gerecke.
Unfortunately I haven't worn it much as I've found it's a bit too slack for me.

The cage diameter is too big, the ring (45mm) might be too big and my balls hang low so when I start getting erect, the cage slides an inch further down my cock and when I shrrink again, I can pull myself out of the cage easily.

I'm planning on buying a hinged 40mm ring from Gerecke however with how low my balls hang, I'm not sure how much of a difference this will make, so I'm considering selling my existing Twister 2 and buying a custom made cage directly from Gerecke with 30mm diameter rather than 38mm diameter cage, hopefully this will grip my cock and stop the cage from sliding away so easily and also stop me from being able to pull myself out.

However I was wondering what length to get (the current cage from Tickleberry is 85mm), do I measure it based on how low the ring hangs on my balls currently or do I measure it from the base of my cock as I'm not sure yet if even with a tighter fitting cage and ring, it might still slide an inch down my cock due to my low hanging balls not stopping the ring sliding down. Ideally I'd like a length that provides almost no opportunity for erection.

I know pierced solutions are more secure but I definitely don't plan on going down that route. Any opinions greatly appreciated as I wanna start wearing a device full time so my KH can have full control ASAP.