This PWC article looks at how market sentiments can impact on private equity valuations. While greater uncertainty in the market does seem to be an issue for establishing sound valuations, the author does present some suggestions for dealing with this.
The second link is the International Private Equity and Venture Capital Investor Reporting Guidelines from 2012. This effectively being the IFRS equivalent for P.E and V.C firms, so it tends to be a bit lengthy. It is interesting to note however that there is a large focus on internal rate of return as this is “the most common measure of performance within the private equity industry”.