Most bike's rear suspension and swingarm setup causes the chain slack to lessen (chain gets tighter) when the rear compresses... ie someones weight is pushing it down. Instead of pulling off the rear shock to bring the rear down enough... have someone else sit on the bike (similar weight as you) then adjust the chain slack. Or carefully... with bike on the side/kick stand, lean over the rear seat (your belly on the seat) and put all your weight on the seat so the rear squats... then with your hands check the chain slack. Works for me!

Cheers!