No problem, @Soupy. I reckon that page is confusing, which is why I explained instead of simply pointing you to the documentation. (Perhaps I should have done both.)
The thing to remember is that when IM needs width and height, the format is WxH where “x” is the separator, and W and H are either numbers or %[fx:…] expressions.
(I don’t have an IT degree, but confess guilty to many years with IM.)