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.