Arm length calculation help on own-design delta needed

Hi All, I'm building a delta and I've got to the stage where I need to cut the arms to length. On my design, the distance between towers is 268mm and the rule of thumb seems to be 80% of that distance, meaning 214mm. Before I start cutting tubes to make the arms, would anyone with experience mind double-checking my logic?



Hopefully the diagram above makes enough sense to go on, please let me know if there's any dimensions that would need adding

Thanks

Barny