absolutely not, it's just a guide for an approximation. every blade will vary a little on their circumferance, even two identical from the same manufacturer. I tighten the blade until there is no deflection when I put a little pressure on the side of the blade
I've had other band saws and always tightened them by the same method and the only one that broke was a 1/8 inch blade that my craftsman saw wasn't designed for.
I would suggest you getting a 3/4 inch blade which is the maximum your saw is designed for and only use it for resawing. With the cost of wood these days, I dont want to waste any so If I need a piece of wood 3/8 inch thick I resaw a piece of 3/4 and save the cutoff and eventually will glue two or more pieces together to get a thicker piece. No, I'm not scotch
I also would get a 1/4 inch blade if you're planning on doing any curve cutting, for really tight curves, then you'll need a 1/8 blade, all of which your saw will handle, so long as you tension the blade right and let the saw do the work
If you see smke when you cut, then you're hogging the cut, slow down