Sorry it took so long to reply, working some crazy shifts this week. Yes I see your levels in your post.
A SLAM takes as long as needed to kill whatever is consuming your chlorine. While the water may look clear, if you cannot maintain a FC level there is something organic being oxidized, most likely algae spores and bacteria/virus cells.
To keep the time to complete the SLAM as short as possible, it will require that you test and dose chlorine as often as possible to maintain your shock level FC, this could mean once an hour at the start to get your FC to hold. After that first day you should notice that you can go farther between doses. A SLAM is complete when you lose less than 1ppm of FC overnight, your CC is 0.5ppm or less, and the water is clear of
any algae, dead or alive.
The SLAM also requires that your filter run 24/7 during the SLAM.
Pool School - SLAM - Shock Level And Maintain
As as far as pool math is concerned, if you refer to the pic I posted you will see that I put your pool's water capacity in gallons in the box at the top left, in the 'size' section. Directly below that is the FC section. The box on the left (the 'now' column) is where you would enter your current FC result (in the picture I entered 0). In the box directly next to it (the target column) you enter your target FC (in the picture I entered 20, your shock level). To the right of that box is where pool math will tell you how much chlorine to add, but be sure to enter the proper chlorine concentration (in the pic I entered 10 for the trade %). In the pic I posted this worked out to 384oz.
In my post yesterday at 11:51am I gave you the amount of chlorine needed to raise your pool 1ppm (19oz). If your SLAM shock level is 20ppm, and you get 8ppm from your test, you need to add 12ppm to get back to shock level (20ppm target minus the 8ppm reading = 12ppm to add). This will require 228oz of chlorine (19oz X 12ppm = 228oz of chlorine needed, about 1-3/4 gallons).
I hope this helps,
Dom