Replying to Instep height?

so I got fitted 'professionally' and so I have a snug fit around the base of my foot and have a custom footbed.

How much room should you have in the instep? When my foots flat, I have a solid inch, or almost exactly an inch Above the top of my foot right before my ankle. My heel often lifts up when landing heavily switch, and I have mad shin bang currently from skiing hard 3 days in a row.

so, how much room should I have in my instep? Do I fix this by adding shims? Will I be able to add shims myself?

Note im riding the 3 piece roca free bird. Is this a high volume boot?