Do you have any advice on teaching a dog to sit straight in front on a recall?

I teach fronts with puppies and small dogs by sitting on a stool or bucket. I angle my legs out to form a V and then teach them to look up at my face to get a reward. I gradually use taller and taller stools and move my legs to a more normal position as they show they understand. How fast you move to a normal standing position depends on how quickly the dog grasps the concept.

You can also use visual guides on the floor, like pvc poles for the dog to position themselves between.

I have also used a small touch pad that I place in front of me in the desired position, if the dog knows touch pads this is very clear and the biggest problem is fading the pad. I use smaller and smaller touch pads until the dog doesn't need it anymore.