The closure of the Athens Stock Exchange <.ATG> since Monday has made it impossible for managers to ascertain exactly how far the value of shares in bluechip Greek companies such as Alpha Bank (>> Alpha Bank S.A.) and Hellenic Telecommunications (>> HELLENIC TELECOM. ORGANIZATION S.A.) might have fallen since the country fell deeper into crisis. It is unclear when the bourse will reopen.

Only a handful of fund firms still hold large volumes of Greek stocks, after years of deep recession, political turmoil and the increasing risk of ejection from the euro sent all but the most bullish of investors fleeing for the exit.

But even those with tiny proportions of their portfolios invested in Greek companies have been forced to choose whether they use 'stale' prices from Friday's close, or consider special measures to estimate share prices, in order to publish net asset value (NAV) and ensure the fund can stay open for business.

Without an accurate NAV, which is calculated by dividing the value of the funds assets after fees by the number of units held, units can be mispriced when investors look to buy or sell.

"It's necessary fund managers follow the rules governing valuation, which mean that if they have any doubt a valuation is reliable, they must exercise best judgement in valuing all the assets," Mark Sherwin, senior adviser, financial reporting, at funds industry trade body The Investment Assocation.

"The result of the judgement may be different for different managers but it's about recording that judgement in a clear way, and being able to demonstrate how the judgement was made."

Inevitably, some managers are likely over-estimate the hits, while others are likely to underestimate.

One insight into how investor sentiment towards Greek quoted companies has changed in the past week is the performance of the only Greek-focused exchange traded fund that is still trading globally, according to Lipper.

The $304 million Global X FTSE Greece 20 ETF fell nearly 20 percent on Monday but gained over the next three trading days. The ETF is still down about 8 percent from its last Friday's closing price.

STALE PRICES

Only 36 of more than 3,000 equity mutual funds and exchange traded funds investing across Europe had invested 2 percent or more of their assets in Greece, according to their last disclosed portfolio tracked by Lipper.

But for some, the decision to use stale prices or estimates to report NAV, could have bigger consequences for investors. Generali Investments' European Recovery Equity Fund, that bets mainly on euro zone stocks, had invested 14.4 percent of its 631 million euros ($699 million) in assets at the end of May in Greece, according to its monthly factsheet. Other funds with exposure to Greece included Chou Europe Series A, which had 15 percent of its assets in Greece, and UBAM European Opportunities Equity fund that invested 11 percent of its in the country at the end of March, Lipper data showed.

A spokesman for Generali Investments said the European Recovery Equity Fund was open as normal but he declined to disclose how its managers were calculating the fund's NAV.

Hedge fund Horizon Capital Management told investors on Tuesday it had suspended NAV calculation, subscriptions and exits from its funds because it could not value Greek assets, which accounted for nearly a third of the fund.

The $100 million Kairos International Sicav - Small Cap fund , which has investments in Mytilineos Holdings (>> Mytilineos Holdings S.A.) as part of its less than 5 percent exposure to Greek assets, is using last Friday's closing price to calculate its NAV, a spokesman for the fund said.

Alexander Schindler, member of the executive board of Germany's Union Investment and president of the European Fund and Asset Management Association, acknowledged that managers faced a "real challenge" in valuing funds accurately.

"The last valuation was on Friday last week, then we had this downturn in the markets. If you have a European equity fund with 2 percent in Greek stocks, you have to protect those investors in the fund, as well as those who want to buy," Schindler told Reuters at the FundForum event this week.

(Additional reporting by Carolyn Cohn; editing by Anna Willard)

By Sinead Cruise and Nishant Kumar