I dont think that theroy is correct.
Im 72kgs and ride a 123L by 29 wide and find this board perfect for me and what I want.
(Have a windsurf board at 71L and would have zero chance of SUPing it. Its 22 wide and 4.3 thick.)
Someone weighing 120kgs I reckon would struggle paddling my board. Its not if it is possible or not. Its just not gonna work.
SUP surfing is all about getting onto waves by standing on top of a board and powering with a paddle. Glide is the key. Correct? If you are too heavy for the board you will sink the board and start pushing water and more than likely fall in.
Its not a calculation as such. Its the board shape and rider ability. Thats what gets you onto the wave.
We are all different