Advice sought: Shimano derailleur capacity
I was hoping to put an 11-28 cassette on my trusty old sequoia which has a long cage tiagra derailleur and currently runs on a 12-25 block. Having done a bit of homework, it looks like it can only handle a maximum of 27 teeth on the back.
Can anyone please advise, are Shimano technical specifications absolute, or is there the leeway I need to install 28 teeth at the back.
Any advice gratefully received.