Problem adding drawer head

[David Egnoski]

I'm helping a friend build his seed cabinets and came across an issue adding drawer heads. This is a three drawer stack. The two bottom heads are 12.0625\" high. The bottom head is flush to the cabinet bottom. There is a .125\" gap between drawer heads. The top head should be 6\" high with a .125\" reveal at the top. Add all those values and you get 30.5\", the height of the cabinet box. When I try to add the top head, it wants to make it 6.0625\" high and make the one under it 12\" high.

I've never come across this until now. I'm stumped and open to any suggestions.