Jaysef,
If the ball is at the 23 1/3 yd line, I would step off the number of steps to the 25, then go to the 40 and step back that many steps for a 15 yd penalty. It looks pretty sharp and is a lot easier than trying to add and subtract fractions and then apply that to your calibrated stepping (I work with a bunch of engineers, I know how you guys are
)..and as always check with your wings to make sure that everyone ends up in the same spot....