There are some major limitations regarding 1RM formulas:
- They are designed with a certain demographic in mind. If you are outside that demographic, they may not be applicable. For example, the one you listed is designed for men in their 20s.
- They are designed to work within a certain rep range. For example, the one you listed becomes grossly inaccurate beyond 10 reps. The fewer reps involved, the more accurate they become.
- They are only an approximation. This is why they don't work outside of their designed parameters. It's better to think of it in terms of equivalent effort than it is to think that just because you lifting 100kg 10 times that you can automatically lift 133kg.
With those disclaimers out of the way, I can say without a doubt you are using the the formula incorrectly. The formulas become more useful when you understand about the different adaptations that happen with rep ranges. Granted, even the rep ranges are only useful to a point--but it helps to understand how to design your work around your goals:
- 1-3 reps: 90+% effort, used to test strength. Can also be used to primarily stimulate myophibrilar hypertrophy (more contractile protein pairs per muscle fiber) if used with 85-90% and more sets.
- 4-6 reps: 80-90% effort, used to build a balance of strength and size.
- 8-12 reps: 70-80% effort, used to primarily build size and commonly used by bodybuilders. Primarily stimulates sarcoplasmic hypertrophy (more energy system support for the muscle).
- 16+ reps: <70% effort, used to build muscular endurance.
Performing over 100 reps with any weight is a grueling exercise in endurance, and is very difficult. However, that effort does not translate to increasing your ability to do work. I.e. getting stronger or improving your energy systems.
Just as important: warmup should just be that. Getting your muscles warm enough to handle the work load. Performing massive sets with a weight only 2kg from the work weight is going to pre-fatigue the muscles so that you are robbing from what you can do at the work weight.