I've been playing with securing WHMCS as much as possible but pretty much given up on using https with anything other than the admin directory. It seems Cpanel pushes both https and http traffic through port 80 so a simple detection via Apache or PHP only causes an infinite loop. Also since I was asked to temporarily redirect an "admin" folder to the new admin page I have included that below as well.
Here is what I ended up using for my install.
# SSL Admin Folder
RewriteEngine On
RewriteCond %{SERVER_PORT} 80
RewriteCond %{REQUEST_URI} myadminfoldername
RewriteRule ^(.*)$ https://my.domain.com/$1 [R,L]
Options +FollowSymlinks
# Redirect Admin Folder
Redirect /admin/ https://my.zigzap.com/myadminfoldername
Redirect /admin https://my.zigzap.com/myadminfoldername