You want the market to tank on the day you make your paycheck contribution - and dramatically rise with the sun on the next day
The way you split your contributions is dependent on how much risk you are willing to take. That risk changes with the price (value) of the funds over time - but all that stuff can be complicated.
For example, most now find the 'F Fund' to be 'overvalued'. They think it is in a bubble like the tech stocks of 2000 and housing of 2007. Maybe not that bad, but kinda bloated...
As another example, in March of 2009 the 'C Fund' was priced at about $7 bucks. We call that 'undervalued'. Yummy, very yummy. When the market gives you that kind of value (the funds are 'on sale') bump up your paycheck contribution and buy lots of C/S/I.
Anyway, I would recommend setting your contributions to:
40% C
30% S
30% I
Then, when you get a little nest egg, think about rebalancing your TSP asset allocation to something you feel comfortable with. I generally recommend two books to help - Ric Edelman's 'The Lies About Money' and 'The Truth About Money'.
Some bubbas do it a bit different. They use their paycheck contributions to buy the slow movers (G and F Funds) and then rebalance their 'holdings' (what they have in their accounts) as they see fit. I would save that for later. You don't want to be sitting around ten years from now with thousands of dollars squatting in the 'G Fund'. That is a certain 'Alpo Meal Deal Retirement Plan'.
Good luck and happy hunting...