Interface: Framerate Locking

You can use XivAlexander to specify a very specific FPS values. Use Configure > Lock Framerate to access this window. This window also will show the most GCD drifts.

You will probably want to disable in-game FPS limiter from System Configuration.

Automatic Mode

Tick Automatic, and enter values to Target Framerate Range and Render Interval Deviation.

XivAlexander will look for the best fps value within the range you've specified that will minimize gcd drifting. Try out different values for Render Interval Deviation to reduce the chances of your GCDs lapsing to next frame.

Manual Mode

In conjunction to above, specify Global Cooldown value that you want to optimize framerate for, and then click on Adjust Framerate to Minimize Cooldown Clipping to calculate the best value.

Otherwise, you can manually enter values into Render Interval or Framerate. You can check the current values from the main control window.

Examples

Jobs with static GCDs

Best use case for using Automatic.

Jobs with dynamic GCDs

Avoid using Automatic for this one, as drift fluctuates pretty hard.

Recommendation

You may want to stick to Synchronize Processing to Cooldowns and Animation Locks instead.