You can use either of two parameter in a macro to semi-auto cast ground target spells.

/cast [@player] Flamestrike
... will cast it centered on the mage.

/cast [@cursor] Flamestrike
... willst cast it centered, wherever the mouse cursor is currently.

There's a third option that can be realised with ISBoxer to actually place it where you want, explained here:
https://www.youtube.com/watch?v=Qj37aRY3cpA