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Thanks for the great explanation.

My current house has baseboard heaters, 4,000w (13,648 Btu)for the 20x20 room and those heaters do not run continuously, I found it astonishing the Calculator I linked too suggested I needed a 24,000btu.to heat it.
One more method of sizing a mini split is adding one the equals your current houses heating systems Btu output. Although that would assume your heating system is not grossly overkill.