Yup, correct indeed. I would put a fuse on terminal 5. If you don't want it to boost past the mains voltage, then your mains in 'hot' can be moved to terminal 1. Otherwise your good to go.
This is assuming the rotation to increase the Vout is CW ('up' in your diagram)
That's a nice variac as it can be wired to CW or CCW voltage control. Not all can do that. I have these Staco guys here (6510's I think, 25A, 0-180V) that are made for CCW voltage increase so I had to turn them around so the wiper is close to my hand.<read ZAP