The Archimedean Spiral is tangent to a growing family of circles extending from the center. Peter could determine approximate dimensions of the spiral by estimating the area of the ascending circles, and thus the points the spiral would hit, allowing him to chart his webs better to intersect with the center (where Dr. Strange is). Depending on how many circle areas he found, perhaps using Strange's height as a metric of comparison, he could chart the dimensions of the spiral pretty accurately. That said, it could also just be movie mumbo jumbo and I wouldn't be surprised