8 meter arc hash mark distances

I am a grounds keeper for Saline High School in Michigan. We just started up a girls lacrosse league and I'm lining it for the first time. After reading the field layout information and looking at diagrams I am still confused about the 8 meter arc hash mark distances. I worked off my center hash mark 13'2" to position the other hash marks. To do this I used my tape measure and laid it directly on the arc. Afterwards I reviewed my picture again and wondered if I was supposed to use a straight 13'2" line to measure over from the center hash. )- The hash marks will be closer together if I measure 13'2" on the arc and further apart if using a straight line to intersect the arc. Should I be measuring on the arc with a curved 13'2" line or across the arc with a straight line.

ANSWER
Take a look at the rule book here:
2010 Womens Rule book

Page 12 of 108 spells out how to line it pretty clear. If you are still having trouble let me know, and we will try again.

good Luck,

Jamey