The amount earned from forging is equal to the amount of fees included in the block.

Since the minimum fee is 1, you will earn at least 1 coin per transaction in the block. Sometimes there may be 5 transactions (mimimum 5 coins as a fee), there may be 200 transactions (minimum 200 coins as a fee), or sometimes there may be 0 transactions, really depends on timing/luck.

It's difficult at this time to use a calculator or other tool to estimate the earnings from forging.

Even if your income from forging (at the moment) is low, please leave your wallet open and forging to strengthen the network.

*As a side note, I've been using my forged balance to make lots of transactions with higher fees and hope others will do the same*