You can view http://www.zpool.ca/api/status for the current estimated payouts.  right now blake2s is at 0.00060351 btc/GH/day.  so with 12.55GH/s you'd earn 0.0076 btc/day (~$USD 20), assuming that exchange rate and difficultly holds steady. (which it wont).
0.00060351 * 12.55 = 0.0075740505

but yes, your maths on the 24hr/actual is correct. It's just that the previous 24hrs are not necessarily a good indicator of the next 24hrs due to the high volatility of all altcoins.