If you wonder how to improve your call-up position for nationals in 2018 and 2019, these three useful documents explain the process in which the USAC system assigns value based on the quality of field, points per place and ranking points:
http://www.wsbaracing.org/home/usac-ranking-points-explained
https://www.cxmagazine.com/wednesday-wonderings-followup-confessions-usac-points-chaser
http://www.usacycling.org/usa-cycling-rankings-faq.htm