Did this today. First I worked up to a new 3RM of 225...I had not done front squats since May..and I topped out at 185 for 3 back then.
Given my newly acquired 3RM, I guestimated 60% to be 135. So I did 135x13. I left some reps in the tank too, so I shortchanged myself.
I might give this another go in a few weeks.