If there is no play in the headset when rocking the bike against the locked front brake, the headset is tight enough. Modern cartridge bearing headsets can take a fair bit of preload without binding so that's not a good criteria for adjustment. There is never any benefit in having the headset bind as long as there is no play or slop.
As a check, be sure the compression plug hasn't moved up under the adjusting bolt's tension and be sure you have a couple of mm gap between the top of the steerer and the top of the stem or top spacer. If the plug has slipped or the steerer is flush with the top of the stem you will not be able to adjust the headset any further.