Property sizing mini-splits

I'm trying to purchase single-zone Mitsubishi Mini-Splits, but after a few quotes, the HVAC contractors, including Mitsubishi's approved contractors, seem to be randomly estimating sizes with each of them planning different sizes. According to customer service agent at Mitsubishi, the rule-of-thumb is 30 X # of sq ft = BTUs, which would be 12,000 BTUs for a 410 sq ft room. However, that differs greatly compared to an online load calculator with the result of 24,000 BTUs required.

Here is the room info: 410 sq ft, including living room and dining room areas that are partially separated by 4.5’ walls extending from each end of the 16’ width, leaving a 7’ opening in between. The house is nearly 100 years old in Southern California, has poor insulation and the room has south-facing windows.

It seems the choices are 12k BTUs X 1 or 12k X 2 or 9k X 2. It would be great to get suggestions for how to prepare a reliable load calculation, for the optimum size and number of units and whether or not single-zone units a good choice for a total of 3 or four units covering the whole house.

Also, there are 2 bedrooms at 130 sq ft each and I plan to go with single-zone 9,000 BTUs each. Since the minimum capacity of both 6,000 and 9,000 BTU units is the same (1,700), I figure it’s best to use the bigger one.

Thanks.