I use a cheap carpenter’s laser that my nephew gave me that looks something like this one,
![](https://i5.walmartimages.com/seo/Mini-Vertical-Spirit-Level-Tool-Laser-Level-Laser-Guided-Level-Line-Measurement-Gauge-Tool-1PCS-Red_ee33d729-68b6-4c77-bc56-2354e82d8615.ab7c1107d36364c0d706ff82f087af19.jpeg?odnHeight=768&odnWidth=768&odnBg=FFFFFF)
I just center it atop my monitor and eyeball it while aiming at a pillow where my head would be, using the top of the sofa as a gauge. I used to be fastidious about it but just gave up by finding that increments of about 2" suffices for my tastes.
I could not discern much of a difference going any less than that. It could be dependent on the horizontal dispersion of the speaker that accounts for that as I remember having to be more picky using smaller tolerances with other, older speakers.
All the best,
Nonoise