After flying many years for a major airline I am faced with the task of estimating my night time and cross country time. I have over 16,000 hours. I need this information for my resume as I am planning to fly past 65 years of age after flying with a part 121 carrier. My company online flight log does no break out night time and cross country time.
There is no Federal Aviation Regulation that provides for estimating flight time or specific types of flight time like night or cross-country.
There is guidance Here for FAA Inspectors to use with respect to lost pilot logbooks. However, I don't believe this particular guidance will provide you with useful information on how to estimate your night or cross-country flight time based on the specifics outlined in your question.
Undoubtedly, you are not in need of flight time in order to meet a specific pilot aeronautical experience requirement for a certificate or rating.
If I were in your situation, I would likely complete my resume similar to what you have said in your question and note that you are making a reasonable estimate regarding the night and cross-country flight time that is included in your total time of 16,000 hours.