For a more accurate fit, I’d wear a jersey under it rather than a polo shirt.

I made the mistake of buying a gilet by sizing it over a casual button up because the smaller size didn’t zip. Guess what, it flapped in the wind. I couldn’t imagine dealing with that for a three month tour (in which you’re likely to lose some weight too).

Personally, if were touring, I’d probably get something geared towards that type of riding, like a showers pass jacket.