According to a site [1] pointed to by another post in this page, the estimate for "Minimum Orbit Insertion Distance" was about 0.02 AU or about 3 million km (in round numbers).

Luna's distance is roughly 384,000 km -- so call it 8 lunar orbital distances...

(edited to add the link) [1] https://www.projectpluto.com/neocp2/mpecs/ZTm0038.htm