It is easy to calculate the block rewards because it is static (6.25 btc/block), the rest comes from the transaction fees which varies greatly and there is no way to figure out future blocks rewards, you could do something like taking the average block fees for the past month/year and get the average in %, if block fees are 10% then simply multiply your number by 1.1, but again all of these are rough estimations as long as you can't predict the future accurately.