I agree with you SafetyPee. This scuttlebut about adding margins to Vref for gusts has been around since Pontious was a pilot, but so far as I know is unsupported by any formal evaluation or research.
My experience is that similar margins have value in small aeroplanes with a high drag / low inertia profile. However I don't believe that this is particularly sensible practice in the majority of cases.
Rans -can you give a reference for these factors you're using - I have a suspicion you may be looking at something derived from barside banter, rather than certified operating data.
G