I know actually there no way to add his own custom ratio in Photo Studio 6 for Mac ( I have opened a topic for that issue ).
But at first I thought I could at least use Width and Height to define a one shot custom ratio, but even not !
For now my workaround is to use external editor to crop picture.
I think ACDS should try to use crop function in Pixmaltor Pro to get inspiration.

I don't want to be too negative, there is a lot of things I like in ACDS ( especially the file manager ) but disappointed by some basics functions like cropping.