dont use what your "street shoe" size is. Go to any shoe store use one of those metal foot size measurements and then get your true shoe size. From that go find a mondo shell size to US shoe size chart and pick whatever matches.
But your best bet is to go to a boot fitter and have them tell you whats best. But if you live where I live and the boot fitters are completely clueless I did the first method and it works for the most part. Just figure out what type foot you have, narrow, wide etc. Find a company that tailors to your said foot dimensions use what i sad in the first chat and just buy some boots.