3 wraps using 1.75 as both the main line and prusik will slip between 25 and 30 pounds.
3 wraps using a 1.75 prusik on 2.2 main line will slip between 35 and 40 pounds.
I use 3 wraps on my guy lines and TRL prusiks other than the figure 9 prusik in which case I use 4 wraps.
Ike, that prusik looks far more involved than is neccesary. Slip is not the enemy in your tarp cordage. Unless you would rather tear up your tarp as opposed to being bothered by having to re-tension a line.
Opie, do you see any problem using 1.75 for the TRL?