Wow, 4 days is fast for water to turn green if the FC was increased before you left. Well, no worries, now you need to SLAM. Things to do now:
1 - Lower pH to about 7.2
2 - Because your CYA is rounded up to 60, increase the FC to "24" and keep it there as much as you can until you pass the 3 SLAM criteria (link below). Make sure to follow the instructions on the SLAM page carefully.
As for the calculator, do the pH first, entering your now value (8) and a goal of 7.2. Next is the FC. Enter your current FC (3.5) and then your goal of "24". There's a "WEIGHT" field for percentage, so make sure to manually enter the strength of your chlorine. If it's regular household bleach, it may be 8.25%, but double check.
That's it! Test as often and increase FC to stay at 24 and you should be on your way. Good luck!