So you want to learn how to calculate the PoS block validation rewards (staking rewards) when staking in the PoS Finality Chain?
Staking rewards depend on:

Circulating Suppy (
C
): Total circulating supply of CFX. This can be obtained from sources such as CoinMarketCap. 
Staked Supply (
S
): Total amount of staked CFX by all validators. This quantity must be estimated. A good backofthenapkin estimate is to sum all of the staked supply from public pools. 
Base Interest Rate (
i
): Fixed base interest rate, fixed at 4%. 
Staked CFX (
stakedCFX
): The amount of CFX that you stake.
You can estimate the staking rewards in the PoS Finality Ledger, with the following formula:
PoS Interest Rate = sqrt(C / S) * i
Annual Staking Profit = stakedCFX * PoS Interest Rate
Let’s do a test run!
Say there’s a circulating supply of 1.43B (at the time of this post) and there’s currently 50,000,0000 CFX staked across all nodes.
Our PoS Interest Rate
would be:
PoS Interest Rate = sqrt(1,430,000,000 / 100,000,000) * 0.04)
, or 0.1513
.
This means that we would get a 15.1261% of interest rate per each staked CFX.
Now, let’s say that we want to stake 10,000 CFX. Our annualized staking profit would be Annualized Staking Profit = 10,000 * 0.1513
, or 151.2614
CFX.
As you can see, this a slightly more complex mechanism to estimate staking rewards than the governance site, but can yield greater profits. I hope this helps out!