A couple suggestions from the voice of experience.
1. Vacuum to WASTE as much "****" off the floor of pool. It will only make it harder if you keep it in the pool
2. Fill the pool back up
3. Test your water, write down your answers
4. Add two gallons of liquid chlorine in front of the water return with the pump running
5. Re-test your FC levels.
6. Using the Pool Math tab, set the chlorine setting for you chlorine strength and use the "Effects of adding chemicals" section to see if your gallons guess was close.
As an example, if enter 10,000 gallons at the op and enter that I poured in 128 oz (2 gallons) of 8.25% bleach, i should see an increase of 8.5FC. If my testing showed the FC went up 5, my gallons guess was low. Play with the gallons until you see the "Effects of adding chemicals" section gets close to your results. While this is slightly inaccurate, it will get you close enough to start a real SLAM.
7. SLAM using your new gallonage and the Pool Math calculator.
8. Watch you filter, if the pressure gauge doesn't change (increase) open the filter up anyway and look (and consider getting anew guage) . If the pressures do rise, clean or backwash as needed.