Do not think about this to much,take your time,one stroke on a strop is one trip up and down,speed is not a factor,forget about it.
Concentrate,one up, one down,take your time,slow down,if it takes 10 minutes to do 50 strokes,so be it.
You need to learn a cadence,slow down,proper stropping is paramount,it is critical,slow down,do not push the river.