For Kyogre and Groudon, and 18% reduction, and Rayquaza 9% is sufficient to bring them under 4K.
For Slaking, a 27% reduction, but I don't think a stat reduction alone is the right course of action for him. Even at around 4K, it would be extremely overpowered for a regular (i.e. non legendary) pokemon. Something that replicates the ability Truant would be necessary to do Slaking justice.
I personally don't think the nerfs to Mewtwo and Ho-oh were necessary. It's not like they were gonna overwhelmingly dominate the current meta with their original max CP (considering legendary Pokemon can't even defend gyms, anyway) when there really is no meta at the moment. But, I digress.
If the big three Gen 3 legendaries (and Slaking) were hit with the same stat reduction as Mewtwo and Ho-oh, I think:
-Slaking would be slightly above 4000 CP, maybe around 4500 CP
-Kyogre and Groudon have equal stats (both have a max 4874 CP currently, just about 100 CP higher than Mewtwo's original max CP), so both would barely reach over 4000 CP
-Rayquaza's current max CP is 4354 CP, so its new max CP would most likely be somewhere between Lugia's and Tyranitar's max CP
Slaking would probably require a higher percentage than 9% to be under 4000 CP, and maybe about 11% for Groudon and Kyogre.