Use a 40mm ring size for holy trainer II. Would I need same size oval ring for jailbird in anybody’s experience switching from HT to JB?
No. It’s completely different. The Holy Trainer shapes fit like no other base ring. Yes, go talk to the manufacturer for advice. I got lots of advice from Mark at MaleChastityNow on my Contender fitting and I finally had a comfy cage I could wear 24/7 and actually get some sleep! I do prefer the round stock base rings that MatureMetal and MaleChastityNow use. There’s no poking from stabilizing posts either. I have moved to an oval ring to modify the gap between the cage and base ring. To address your question directly:I recommend finding the **circumference** that you can wear comfortably and not get skin folded over and pinch or burn. Find the equivalent circumference and you may have a shot at nailing your base ring diameter. mind this: the Jailbird is heavier than the HolyTrainer. I’ve found I needed a tighter ring to carry it. Some go with double rings. In my Contender, it’s lighter by design than the MM JB, but also mine is short and straight and never draws down due to weight. As such I don’t feel I need to step down below 1-3/4” ID... that is a 5.5” or 140mm circumference, for me to keep it from dragging down. I felt the HolyTrainer kept “popping up” a lot when hard. It felt weightless. And at night it would always either slip off, or I needed it so tight I felt pain. Regardless. Don’t worry. You’re making a good investment going to a custom made cage. Don’t skimp out on the full user experience by trying to order without working with the custom chastity maker. Don’t hide behind the web form.
I have worn the 1 3/4" base ring for most of my devices including the Watchful Mistress from Mature Metal with no problem. So when I ordered 3 different SS ones from EBay I went with the 45mm which is basically 1 3/4". I've found now that I should have ordered the 40mm ring which I just did and they are on the way!