For powerlifting, it's very easy to calculate your 1RM. There are plenty of calculators online and all you need to consider is the weight of the barbell.
However for weighted calisthenics, you must also include your bodyweight in addition to the added weight since they are both moving.
Kensui, who makes the Duo Gym Rings I use, made a 1RM calculator you can use to calculate your 1RM when doing weighted calisthenics