I cannot speak for Serenata's calculator, but the best strategy I've found is to invest exponentially. Put 1 coin in your first account, put 10 coins in your second account, and put 100 in your third level account (So that the 1 btc account gets the 10 btc account as a referral and the 10 gets the 100 as a referral). I expect no one else would want to share this because it means a smaller referral bonus for them
Math notes:
Returns can approach 20% (instead of the original 10%) by investing this way, however using ten times as many coins at each level is less effective than 100 times (even for the same amount of total coins). So investing 100 coins it would be better to do 1 & 99 than 10 & 90
And obligitory links for any takers:
http://pyramining.com/referral/7p8sey4b9 2012-08-13 15:13:30 UTC still active
http://pyramining.com/referral/tyr9kacns 2012-08-13 15:13:30 UTC still active
http://pyramining.com/referral/t2cqfg3ka 2012-06-10 19:14:53 UTC still active
http://pyramining.com/referral/dzfngc2b4 2012-06-10 19:14:52 UTC still active
http://pyramining.com/referral/ybt3xs6q7 2012-06-10 19:14:52 UTC still active