I have used the formulas presented in this link and slight variations or tweaks to enhance imaging and sound stage. The addition of room treatments have helped also.
Good jumping off spot and you will get to your goal on vocal imaging.
Dependent upon speakers, rest of system, toe in, room and room treatments, recording. But generally, I've heard center image vocals as mostly in or behind plane of speakers, only hard panned vocals sometimes extend in front of speaker. This has been a constant with box, open baffle and horn speakers.
The XLO disc is good for that image placement track.
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