Another delta arm length question

Looking for an optimal arm length for my delta build? Some people says that with the effector in the opposite position to the extrusion the angle should be minimum 20 degrees and 25 degrees should be optimal. I also heard that Diameter*1.2 is something to use.
The bed is 500mm in diameter and as the design is now the arms can be straight down (vertical) when the effector is in the closest position to the extrusion. I use this formula to calculate the arm length L=D/COS(a) and with that I got the following:
20.0 degrees = ~532mm
22.5 degrees = ~541mm
25.0 degrees = ~552mmWith the formula Diameter*1.2 = 600mm (~35.5 degrees)
Maybe I'm wrong but when it is possible to have the arms vertical there is no need to use any offsets when calculating?

It looks to me that 550mm would be about right.

@dc42 thanks. Now the design will continue.