I've been looking into EOS for the past few days and IMO the only sensible time to buy in during the ICO would be at the very end of each purchase period. This is the only way to determine roughly what price you will be buying in at.
a * (b/c)

a = Total ETH contributed by an authorized purchaser during the period.
b = Total number of EOS Tokens available for distribution in the period.
c = Total ETH contributed by all authorized purchasers during the period.