Moving in an arc

I want to make a spell like magic missle from Baulder's Gate II. Where the extra bolts come out from the side and move in an arc towards the target. I have a formula to get the arc but it only works if the unit is standing still.

In a quick overview I save the casting units position, the target position, and the facing angle of the caster. I find the midpoint and create a unit at that point with an polar offset of 300 at the facing angle -180, this makes a unit right next to the hero. I then order the unit to move to the same point offset by different angle using a loop so facing angle -170 for example. This moves the unit in an arc. Its not smooth, its done in gui, and it doesnt work if the target unit moves.

I know some jass, ive done mostly editing of other functions but I can write some. I couldn't find a function in the download section to do this type of movement, and I dont believe Lord Vexorians Caster sytems move unit as projectile functions will allow for this either.

This might be better off in the jass section but I think some of the people here could help me with this.