The pivot line is 13 feet wide, the difference between the inside curve distance of 12.5 feet and the outside curve distance offset by the 1 foot variance (26.5 feet minus 1 feet = 25.5 feet) 25.5 minus 12.5 gets you the 13 feet. So between the two players, there's approximately more than 10 or 11 feet.
However when defining proximity, skaters who occupy the same 90 degree plane actually have zero distance between them. This is important to remember when defining packs, because two lines of skaters occupying the inside and outside of the playing field would essentially have no space between them.
The Red Pivot and the Green Pivot have zero distance between them in regards to proximity. As long as the Red Pivot and the Green Pivot remained the designated pack, the Red Blocker would have to maintain proximity to be considered part of the pack, or at least within 20 feet of the pack to remain in the engagement zone.
It's a fairly simple concept, but it can take a lot of work explaining to someone that there is actually zero distance between two players. You could be 11 feet apart on the 90 degree plane, but you might as well be touching each other.
This comes up more from a reffing perspective than a player perspective, because the Pack Refs are defining packs based on proximity and it doesn't take much to change the dynamic of that proximity.
This is also helpful to skaters who are using a slow, deliberate strategy to keep the pack pace slow. By maintaining proximity to another skater, you are still making an effort to maintain the pack, even if she is on the inside and you are on the outside.
FORWARD PROGRESSION: I see this happen a lot, and we've been victimized by skaters unaware that they may be crossing the jammer line during knee-down starts.
Remember knee down starts are utilized to create No Pack designation upon the Pack whistle. With the evolution of derby the persistent use of the knee down start is becoming more prevalent, and skaters are picking up Illegal Procedure penalties for lining up Out of Position.
The main explanation of this came down after Windy City's use of a strategy sometimes referred to as Spiral Staircase, Golden Spiral, etc. Essentially the Windy City blockers were trying to get their jammer through quickly, so they utilized a form of the Killbox, but lined up behind the jammer line - taking full advantage of the rule below.
Skaters who line up behind the jammer line are considered Out of Play as a result of "leaving" the front of the pack. This is because Game Play happens from the Jammer Line and progresses counter-clockwise. See the diagram at left. The Red Blocker behind the jam line is considered Out of Play - False Start upon the Pack Whistle, which is simply a minor.
This is a good way to pick up an intentional Fourth Minor. However, if the minor is in error and it's not the fourth the skater is ruled to have left the pack from the front of the pack, not from behind. If the Red Blocker were to skate forward to rejoin the pack, she would be issued an Illegal Procedure Major for Illegal Return/Re-Entry.
Since the Spiral strategy became wildly popular, Women's Flat Track Derby Association issued a couple of rulings:
1. If at the start of the jam a Blocker is touching on or behind the Jammer line, she is considered to be out of position ahead of the pivot line and has committed an Illegal Procedure False Start minor penalty.
2. It is required that the majority of on the track Pivot Blockers and Blockers from each team begin in this pre-jam positioning.
These two publications help illustrate why it's important to make sure skaters line up in front of the jammer line, not on the line, and also why you want to make sure most of your skaters are In Position prior the the Pack Whistle.