I am not sure, but I think there is no need to take care on timing.
In the example given on the EOS site:
20 EOS Tokens are available during a period.
Bob contributes 4 ETH and Alice contributes 1 ETH during the period.
As a total of 5 ETH were contributed for 20 EOS Tokens during the period, 1 EOS
Token will be distributed for every 0.25 ETH contributed. Therefore, Bob receives
16 EOS Tokens and Alice receives 4 EOS Tokens.
regardless of timing of purchase, the number of tokens you get is calculated after end of each period. And the ETH price in BTC or $ seems also not relevant.