what's your waist size?
normally, suits and blazers tend to have a 6 inch drop in sizing. for example, if you are 40R in the chest, that means you are most likely 34 in the waist. now, there are exceptions to this for example. if you work out a lot and have an overdeveloped torso but a small waist, you could easily be a 10, or 12 inch drop. also, some people who are really fat tend to have a 3 inch drop between chest and waist.
bottom line is, if you are not a bodybuilder, and are not a fat ass, your jacket size is 6 inches higher then your waist size.
however, based on your height, im not sure if you are a short or a regular cut since i don't know your arm outseam. when you buy dress shirts, do you normally buy the ones with a 32-33 outseam, or the ones with a 34-35 outseam? if it's 32-33, go for the short fitting jacket. if you are 34-35 go for the regular. if you don't know what your arm length is, go for the regular and get the sleeves shortened if need be.
if i were you, i'd go to the nearest men's warehouse sometime in the future (or moores if you live in canada), and get all your measurements taken by a suit specialist, and keep the card in your wallet (rather then ordering random sizes).