No first-hand experience, but my rule of thumb* is "tyre width + 10mm".
Guards the same width as your tyres isn't going to leave any clearance.
*Something I read on the internet.
My rule of thumb, which is similar to yours, is as wide as will fit in the frame, but clearly you need to have a wider guard to give clearance.
Sure, the wider the better: I should have qualified my ROT as being the minimum required.
@andyp started
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No first-hand experience, but my rule of thumb* is "tyre width + 10mm".
Guards the same width as your tyres isn't going to leave any clearance.
*Something I read on the internet.