Turret Rotation Relative to Body

Not sure if I'm just completely missing something because it seems like this shouldn't be an overly hard problem to overcome, but...

I've got an object, a mech but it may as well be a tank in operation, made up of two objects- the legs/tracks, and the torso/turret. I want the torso's y-rotation to always be set to the leg's y-rotation, modified by a torso set value which would be modifiable by the player.

So, the torso set might be 35, so the torso is set to the leg's y-rotation plus 35 degrees. Or -22, leg's y-rotation minus 22 degrees etc.

How would I achieve this through javascript? Admittedly my only experience is Scirra Construct, essentially easy mode I suppose, where I could just set the torso's angle to MechLegs.Angle + TorsoSet haha.