"The Customer must own at least one Heavy User license for every two Light User licenses Customer owns."
http://support.ptc.com/support/legal-agreements/pdf/Licensing-Basis-Table-EN-201507.pdf
Licensing Basis
Also, realize that light licenses are only allowed for users in certain departments.
"Windchill "Light User" licenses may only be assigned to Registered Users who primarily work within the following organizations or functional areas within the Customer’s organization ("Light Users"): manufacturing, production; purchasing; finance; quality; sales; service and support; and marketing. However, any Registered User who has substantial duties within the following areas shall not be considered a Light User: product engineering; application engineering; product management; Windchill system administration; program office; technical publications and procurement engineering. Light User licenses may not be assigned (or reassigned) to persons who are not Light Users unless and until the Customer pays the applicable upgrade fee(s) for software and Support Services. The Customer must own at least one Heavy User license for every two Light User licenses Customer owns. "
View/Print Only licenses don't have either of these restrictions. Make sure you really need a light license vs. just a view/print license.