I think youre on the right lines with your calculation but it might be worth emailing the Spesh UK helpdesk to check, that way if something does happen you at least have proof. They are really helpful and will let you know exactly what the warranty limitations are on your particular frame.
I did this when I was building up a stumpy fsr 29er and this was the response:
You can run 140mm forks on your bike with no real issues. 150mm it’s the max you can run on that frame without voiding the frame warranty but you may find the handling isn’t as good. Depending on the frame 10-20mm is the max, which is why you do get the 10/20% on the website (Evo’s for example already have a 10mm longer travel fork so only can take another 10mm max).