Each note that  played by player has 4 parameters.  Position, Length, Position within chromatic scale, and Gap. First three parameters is obvious for musical notation.  Gap shows chance of player to play particular note accurately (in time)

The goal is to play notes accurately in time calculated based on Position. If note hasn't played in time than it does less damage. If note has played in time it does full damage and also give player XP points. More of it, if timings is right Gap parameter changed to make player's chances to play particular note bigger. 

Thanks to Daniel Schroeder I didn't spent much time to understand how to write Gap values in file.

https://blog.danskingdom.com/saving-and-loading-a-c-objects-data-to-an-xml-json-...