Leaning forward during HBP/ PE8 (undergrip single bar dip)

When I do these I can't seem to stop leaning forward over the bar, like I am lowering my chest down to it instead of lowering myself down in front of it. I didn't have the problem with the normal single bar dips, I think because I had been doing those for a while before starting the Foundation program.

 

The video is straight on so it's hard to tell, but what is an acceptable amount of lean and how can I correct this?

It's going to happen to some degree, but will lessen as you get better at the movement. You have to keep your center of mass over the hands, don't forget that. Doing so requires either leaning or piking at the hips with straight legs to some degree.