My setup is 2 buttons.

One I have for healing just my main as he tanks sometimes and looks like this:
/target Elohkitty
/cast Healing Wave

And then to heal other party members I target one with my main and have this:
/target Elohkitty
/assist
/cast Healing Wave

There's a delay sometimes with the second one and it doesn't target the member so you want to hit it twice and it'll be sure to target the party member.