But, if one really wants to make it an unopposed Success Test, I'd say that the threshold should at least be the # of net hits the restraining party last scored to hold/keep a hold on you.
Wouldn't you want it to be actual hits, not net hits? The net hits subtracts out the original roll of the defender.
I could actually see the rule working well with actual hits. On the initial grab, the attacker sets the threshold with their actual hits (the first defense roll by the defender is a kind of "free" Break Free attempt). In future rounds, the defender is trying to get around that initial Threshold; tactically they are trading a major action to try to break free without the -4 grappling penalty. If you worked it that way, then the "Restrain" option on page 111 would be a way for the attacker to try to increase this Threshold, maybe after an initial lucky grab, at the risk of the defender making a lucky roll and getting out of the hold without taking action.
That system makes sense to me. As a house rule, you could rewrite most of that section on page 111 to use a "Grappled" status, and it would be a simple matter to make that a "Grappled #" status, where # is the current Threshold to get out of the status.