Your eye can only focus on one thing at a time. Since you have no control over the target - there's no point in focusing your eye there. You have a little control over your rear sight; you have the most control over your front sight - focus your eye on what you have the most control over.
That means the target will be blurry - that's okay. The rear sight will be blurry - that's okay.
The only way to know if you have proper sight alignment and that you are precisely aiming is to focus on the front sight.
If you're using an optic, focus on the reticle.
Red dots offer you the ability to focus on the target (which is natural); but you sacrifice precision.
Want to improve your ability to focus on the front sight? Practice! Train your eye.
Unload your firearm. Find a blank wall and set up so the muzzle is about 1" from the wall.
Focus your eye on the front sight for a slow count of 5. Close your eyes and relax for another 3-5 sec. Repeat this 10x.
A more advanced regimen - exhale before you open your eyes. Pause your breathing for that slow count of 5.
Vary the ambient light conditions (remember, most firearms encounters are in low light conditions)
Work in a dry fire to ensure the sights aren't moving when you actuate the trigger.
2 min of practice 3-4 times per week will dramatically improve your ability to focus on the front sight.