The 1 rep max is the most the most amount of weight you can lift. It is the true test of strength. Although, it is something that can easily be tested (and also fun), there are a few downsides.
1) It can detract from your training
Performing a 1RM can be tiring and very draining on your central nervous system. It creates a lot of fatigue for very little gains.
Volume is the true secret to increasing your strength. Performing a 1RM could reduce the amount of volume you perform in the rest of your workout.
2) It can cause injuries
Heavy weights put a lot more stress on the joints and muscles than lower weights. Therefore, performing a 1RM will always have a higher risk of injury than sets with lower weights.
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.
**You can argue your body also moves in a deadlift or squat, but it is only the upper half of your body and it is usually insignificant to the weight on the barbell that is being lifted.**
You can calculate your 1RM for weighted calisthenics using the 1RM calculator which takes your bodyweight into account.