Theoretically what payout would I currently have joining the pool with a 1THash miner?
Reward per share = block reward / difficulty
So reward per Thps = block reward / difficulty / 2^32 * 1e12
For one day and current difficulty and pool fee = 2%
25 / 3815723798.8146 / 2^32 * 1e12 * 86400 * 0.98 = 0.1291645 btc per day
Plus: Some educated guesses on how much it would decline over period of time x?
For the near future, your earnings will probably decline exponentially, at a rate of about 20% per retarget.
plus or minus luck...
[edit] ah I see the 0.98 is the luck bit sorry...
[edit2] and as if by magic the monthly luck just went from 0.98 to 1.00 - as though slush were watching - so your daily rate jumps to a whopping 0.1318
[edit3] and the question should really be rephrased as "Organofcorti - what pool do you point your miners at?".