You'll need to apply for a re-entry permit. I had one a few years ago. It's valid for two years.
The 50% time spent in the U.S is for naturalization purposes only.
For visits outside the country of less than 6 months customs has the burden of proof that you've abandoned your green card. Trips more than 6 months but less than a year you have the burden of proof.
In any case, if it's clear that you are not actually living in the U.S or maintaining your intention of living in the U.S you could have your green card revoked.