Novices and newbies discussion

Greetings everyone! This is my debut post. My partner and I are looking into giving chastity a go, and the HTV3 seems like an excellent entry-level option (for me). The main hurdle right now is that $200 feels steep for us to spend on something with so many size options. I know these items generally can’t be returned, which adds to the challenge. Any advice would be wonderful!

I see they have a sizing guide for the rings; I’m roughly in the 45mm to 50mm bracket, but my bigger worry is the cage fit. It looks like they only measure the tube length where the penis sits fully enclosed (which appears to cover just about 60% of the total tube length from tip to tip). Should I take my full overall length and divide it by two to figure out the right tube size? Thanks once more! Any guidance is much appreciated!