The calculation is usually done knowing center-to-center distance of the two current-carrying elements, not their surface-to-surface spacing. But of course, if the former is known, then the latter can be determined.
In the 1980's when scientists were all "fired-up" (excuse the pun) about the "rail-gun", such calculations were child's play considering that they were using magnitudes of about 10MA. While I don't have any info regarding the methodology used, I do recall bus-bar separations of 5/8" to 3/4".
So, if your query is not a trick question, contact:
University of Texas
Balcones Center for Electromechanics
Austin, Texas