Accurately tuning Basal Metabolic Rate (BMR) using actual weight loss data?

Hi!

I set a weight loss target about a week ago which has given me a calorie budget, however, the actual weight loss I'm experiencing (or even, weight gain on some days!) doesn't match up. I am using the default BMR ("Mifflin St. Jeor Equation") but I do have (medicated) Hypothyroidism which could mean that the default is not suitable for me.

I am wondering if it's possible for me to accurately determine my true BMR by changing some numbers around and then seeing if my actual weight loss mirrors the target, or is there always going to be some (fairly substantial?) variance in the accuracy of the weight loss target?

For example, I am considering the following plan:

1. I set my weight loss target to the maximum of 0.91kg/week
2. I set my BMR to a custom value of 1500 calories
3. I set my "Activity Level" to "none" and use Apple Health to report actual calories burned

If I have not lost 0.91kg in 1 week, then I'll reduce my BMR to 1400 calories and repeat each week until I am losing 0.91kg per week. If I lose more than 0.91kg in 1 week, I'll increase my BMR to 1600 calories and repeat until I am losing 0.91kg per week.

Would this work identify my correct BMR?

Thanks,