Your entire foot should feel snug in the boot EXCEPT for your heel. Don’t worry, your heel should always slip in a properly fitted new boot. As you begin wearing your boot, you will find the heel leather will soften, and your boot will then conform to and move with your foot. The slippage will disappear!
The ball of your foot should sit at the widest part of the sole of the boot. Unlike feeling for the fit in your toes (as you would in trying on a shoe), the proper location of the ball of your foot in the boot will ensure a good fit throughout. Consequently, if the ball of your foot sits too far forward in the boot, your toes will uncomfortably crowd in the toe box of the boot.
The instep (vamp) of the boot should fit snugly over the top of the arch of your foot (more snugly than the fit in your normal shoe or sneaker).