For a single mutual fund it is as follows -
Dec 11 2012 10:20 AM
The fund's total assets = $1,000,000
Less the fund's liabilities = $100,000
The fund's total net assets = $900,000
The fund's total shares outstanding: 100,000
The fund's NAV: $9 ($900,000/100,000)
To calculate for a portfolio of such funds, just take a weighted average of the individual NAV's.