When I set up my present workshop some 20+ years ago, I started by setting the top of the vice jaws to my elbow height, thus setting the bench height, and this has proved satisfactory. I then set my "machinery" bench, which is located adjacent to the vice bench, to the same height thus giving a consistent height across the benches, and thus determining the lathe and drilling machine height. This proved to be too high for the lathe, but satisfactory for the drilling machine, so I made a wooden "step-up" about 4 inches/100mm high to stand on when using the lathe. This proved satisfactory for lathe use and no problem at all in use.
Eventually, I increased the height of the drilling machine by 4 inches/100mm and discovered that the operating handles were too high for comfortable use. I now use the same "step-up" as for the lathe, but need to move it across from one end of the "machinery" bench to the other to be able to use it. Somewhat inconvenient, but usable.
It would seem therefore, that for me, an ideal height for the "machinery" bench would be 4 inches/100mm or so below the main work (vice) bench.
One final point. I am 5ft 5in tall. My grandsons are 6ft 6in tall: they simply kick the "step-up" out of the way!
Regards,
Peter G. Shaw