Hi Brian,
Would 1:1.1 or 1:1.4 magnification ratio be in your acceptance range?
There's so much to consider when trying to hit that 1:1 ratio, focus is just a part of it, I'm sure you already know the intricacy needed to get an overall sharp, aesthetically pleasing image but if you can get satisfaction with something less than ideal then master it by working the gear to its limits.