What is the ratio to build VTSAX equivalency?

[growingstaches]

I want to build a Total Stock Market portfolio similar to Vanguard Total Stock Market Index - VTSAX

My 401k is only offering an S&P 500 index fund.

- Vanguard Inst Index Fund Inst Plus - VIIIX (.02 ER)

So I will invest in this extended market fund in IRA's and taxable accounts.

- Fidelity Spartan Extended Market Index Fund - FSEVX (.07 ER)

What ratio of S&P 500 vs Extended market is required to build a Total Stock Market portfolio?
Does the ratio change over time?
What is the clearest way to determine what the ratio should be?

Thanks,
growingstaches

[the fixer]

http://www.bogleheads.org/wiki/Approximating_total_stock_market

I think the answer you get there is 80/20. Yes it is theoretically possible for this to change over time as wealth shifts between small and large companies. This is not likely to happen except in some kind of extreme economic calamity, so you'll know if you need to check up on it.