Thank you for your replies guys, you have been very helpful.
In case someone else may be interested you can see my updated formula below:
bitcoinPerDay = ((minerHashrate * (blockReward + avgRewardFee) * numberOfSecondsPerDay) / (estimatedNextDifficulty * 2^32)) * poolFeeMultiplier;
For reference:
I have used the API from BTC.com to calculate the average block fee reward of the last 8 mined blocks.
The results of my FPPS calculation are now also pretty close to the results of the mining calculator at
https://btc.com/tools/mini-mining-calculator.
The accuracy may be improved by including more than 8 blocks into the average block reward fee calculation. I will be testing this, but for now I am quite happy with the performance.