How do you handle this measurement?

[RadioBlue]

NF rules.
2nd & 8 from A's 37. QB A1 is sacked for a 7 yard loss. After the play, B1 does a little sack-dance over the QB and draws a 15-yard ULC. The 15-yard walkoff would leave A very close to the line-to-gain. How do you properly measure this?

It seems to me walking off 15 yards and then measuring is something akin to measuring after the ball, which became dead in a side-zone, is spotted at the inbounds marks.

Is it okay to bring the clip back 15 yards and measure to the dead-ball spot and see if A has the yardage necessary for a new series?

The only other option I would see is give A the benefit of the doubt and a 1st down because B was being stoopid.