Many of the questions you ask have varying answers depending on how the divorce decree was written or whether it was silent on particular issues. Normally the time frame for determining the value of a 401(k) or other employer-provided investment vehicle is from the start of investing (presuming the couple was married at that point; if not, whenever you wed) to the date of filing for divorce or when the divorce was final (or when the employee stopped working for the employer, if that occurred during the marriage). Unless a specific dollar amount was listed in the decree and it is wrong, you do not need to worry about amending it or anything. You have a right to 50% of the value from that timeframe, and your half of this asset is not diminished in the least by the loan he took out; that's his concern, not yours.
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Usually you outline the time period for which you are entitled to part of the retirement and the judge makes that finding and includes it in the order and you use that order to present to the company that has the 401k/retirement will figure out how much you are entitled to according to the order.
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