Mature Metal discussion

Hi, I’m new to this site and basically inexperienced to chastity. I’m struggling to find a cage that fits me perfectly but in short I’m a grower not a shower and figured a custom chastity cage would be best and am interested in jail bird from MM. However I think my anatomy is a bit of a pain in the butt to make exact measurements but I noticed the sizing rings that they sell on mature metal — are these useful at all? I already have cb6000s, could I not use this device as a measure unit as opposed to purchasing these rings (I live outside of US meaning I’d have to pay for P&P which is why I’m hesitant) and it be just as effective for measuring myself with? or not because the cb6000 base rings are different sizes compared to the jail bird rings? also I’m about 2 inches flaccid/soft/uncut…so what would be the best cage length? 1”? and I wanted to know something: is pulling out possible in these devices? by ‘pull out’ i mean be able to pull your penis out of the cage and if so does that not defeat the purpose of a chastity device? thanks