Market Cap is one way and gets you 99% of the way there. Another option is if you want to tilt or slice&dice you can use the Morningstar 9 grid boxes, setup a spreadsheet and multiply by the % you want in each and sum them together.
I did this, and using your 92/8% numbers, it is a little off (less than a percentage point or so) in some areas compared to VTSAX (pick your poison). So, if you are tilting in other areas of your portfolio, you could use this to reflect that and then pick up some of the more "specialty" funds in the other 1/3 or so of your portfolio. Just an option for a bit more juggling.
And while I have a moderator's attention, why can't we post excel spreadsheets? I hosted this one on my blog, but GRS should stop being so cheap. http://wp.me/a2h40p-6b