IMHO, 1/2" clearance is sufficient. However, you may find yourself increasing this clearance somewhat to facilitate installation and removal of the lower cowling.
David Heal - Windsor, CA (near Santa Rosa)
EAA #23982 - EAA Technical Counselor and Flight Advisor; CFI - A&I
RV-12 E-LSA #120496 (SV w/ AP and ADS-B) - N124DH flying since March 2014 - 555+ hours (as of Oct 2017)!
through June 2018.