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Unfortunately, I do not have good news for you. Everyone knows about that rule, but that is not the only rule. That rule is not for maintaining Lawful Permanent Residency. That rule is for not disrupting continuity of Residency for Naturalization purposes. But that does not help you with abandonment of Residency.
Anyone that is a Lawful Permanent Resident (LPR) and is outside of the U.S. for 180 days or more within any 12 month period (not necessarily calendar year) creates a rebuttable presumption of abandonment of residency. That presumption can be rebutted by evidence to the contrary such as filing U.S. taxes, maintaining a home in the U.S. and paying that mortgage or rent, maintaining a U.S. drivers license, U.S. bank accounts with significant movement, etc.
Someone that has been outside of the U.S. for more than 1 year without first having an approved re-entry permit has abandoned their residency and only in very few exceptions (such as serious illness) can they get it back.
And take a look at this official link that even says that a trip of less than 1 year can still affect your Residency:
So what can you do? You will need to either spend more than 180 days out of every 12 months inside the U.S. or you will need to file for a Re-Entry Permit that will allow you to stay outside for up to 2 years and not lose Residency status. If you need more time, you come back, apply for another Re-Entry Permit and then you can leave for two more years.
ALSO, if you are going to maintain Residency AND continuity of Residency for Naturalization, then you will need to do BOTH:
1) Get the Re-Entry Permit, AND
2) Come back to the U.S. for a short time before 180 days have passed by.
If you do BOTH, then when you come back finally to live in the U.S. and you have 30 months of physical presence in the U.S. (18 months of physical presence if still married to a U.S. Citizen), then you can finally apply for U.S. Citizenship.
Please let me know if you have additional questions and please do not forget to rate my service to you (not the state of the law) as that is the only way that I can get credit for my assistance. Even after you rate the service, I can still answer additional questions for you without additional charge. If you do rate me positively, a bonus is always appreciated. If you would like to request me in the future, just go to http://www.justanswer.com/law/expert-guillermosenmartin/. Thank you!