Its very easy to do that.. I saw another answer saying otherwise but I have calculated all my post rewards in the past and till now I do it regularly.
Yes, because of fluctuation in price post rewards are not constant. but when yourr weekly author rewards is under 1 grand that fluctuation doesn't matter that much.. you will get an estimation around what you will get..
Now to calculate the rewards follow the process I'm giving you bellow.
-Go to steemworld.org/@(your username). This is a website similar to steemd or steemdb but it is a little more responsive and a little slower.
-When the page loads you will see all your stats. Scroll down and you will see two tab named "Coming Author rewards" and "Coming Curation Rewards."
-Now, coming author reward shows every single dime you will get from your blog posts and you comments combined. Click on "refresh" wait for it to finish calculating and then on the right side it will ahow you what you will get in Steem power, steem and sbd and what curators will get on Steem Power.
If this answers helps you in anyway then you are wellcome.