I believe that 506.2 is considered the 100 year flood level. Our bank decided we needed flood insurance to the tune of $1700/year. We had a flood elevation plan done that showed the living area, all mechanical systems and the septic system was above this level and the insurance became $250.
This chart says the lake made just about 506 sometime in the last 30 years.
http://des.nh.gov/organization/divis.../graph2012.jpg
You can get the flood elevation survey done for around $400.