Quick answer
Subtract the combined shelf thickness from the cabinet’s internal height, then divide by the number of open gaps. In a 960 mm cabinet with three 20 mm shelves, four equal gaps are 225 mm each.
Formula and assumptions
Clear gap = (internal height − shelf count × thickness) ÷ (shelf count + 1). Marks start at the cabinet’s inside bottom.
Worked example
A 1,000 mm cabinet with three 20 mm shelves leaves four 235 mm gaps.