offroad, Smarter people than myself should be able to answer your questions but I'm not sure where your coming up with interest gained. To the best of my knowledge we do not gain interest. Our TSP account is dollar for dollar and share for share.
The Autotracker shows percentages not dollars so it doesn't factor your contributions. Therefore your TSP daily, monthly and yearly percentages should match the Autotracker percentages as long as you show any fund changes. There are some spreadsheets that a few of us have put out if your interested.
Good luck.
Bookmarks