Help me think through this one.

[WestCoaster]

B20 intercepts a pass at the 3 yard line and his momentum carries him into the end zone. As he circles and starts to run the ball back, B55 blocks in the back at the ten yard line. B20 gets hit in the end zone and fumbles. The ball rolls into the field of play and out of bounds at the six yard line.

Since the penalty occurred during his run back and then the ball was fumbled, the basic spot would be the end of the run, which is where he fumbled. Since ending the run in the end zone with the momentum exception in play would result in B's ball at the 3 yard line, that becomes the basic spot for this play, correct? I would enforce the penalty from the 3 yard line. Is there anyone here that would enforce it differently?

I have had no luck finding a case play exactly like this one. I did find one similar in Redding's guide, but instead of momentum, the interception occurred in the end zone. Same foul by B at lets say the 10, then he gets hit in the end zone, and same fumble result through the field of play out of bounds. In this play they say the basic spot is the end of the run which would be a touchback. Basic spot 20, enforcement spot 10.

Am I getting this correct?