So with a polymer coated bullet, how do you decide what size bullet to use? A good rule of thumb is to measure your bore, and use a bullet that is 0.001" larger. So if your bore is 0.355", use a bullet that is 0.356". The question now is how do you measure your bore? This can be done using a process you can do at home called slugging.
A slug is just a small piece of soft lead: It can be a cast lead or coated bullet, a lead fishing weight (not steel!), or just a round chunk of lead that's roughly the size of your bore (In any case, it needs to be a tiny bit bigger than your bore). Using a rod, push the slug through your bore from chamber to muzzle, you may need to use a mallet if it's a tight fit. Afterwards, you're left with a slug that has been swaged by the bore, and should have rifling grooves cut into it. The measurement to take with your callipers is the overall diameter of the slug, not the smaller diameter from groove to groove. The overall diameter of the slug will be an exact match to the diameter of your bore.
What's another way to determine the ideal size for your firearm? An equally good method is to simply test various sizing diameters, we offer small sample packs for exactly this purpose. After loading, there are 2 aspects to look at during testing: How smoothly the ammo feeds and chambers, and the accuracy of the loads. In a 9mm, a .358" bullet may give you phenomenal accuracy, but if you have a tight chamber your ammo is having a tough time feeding and seating into the chamber, it's probably not worth the reliability trade off. Likewise, a .356" bullet might feed perfectly, but the groups it shoots may not be as tight. Find the right balance, and you'll find a load which matches your particular gun and gives you the best possible performance.