If you have experience using DSLR cameras, you won't learn anything in this document. Doing the focus on a multi-camera installation is the same as doing it for a single one. The focus is made from the camera, not from the software. Here's how I do it most of the times:
Option 1- When I do the focus using the calibration bar, I go in manual focus. Then, using the live view on the the camera, I zoom to 10x (top right button on the back of the camera). From there, it's easy to simply manually focus on the center marker. If somehow your white square is not in the center anymore, hit the "delete" button from the back of the camera. Make sure you go back to Manual focus once you're done.
Option 2- If you can have someone standing on the center spot of your installation, then it's easier to use the automatic focus. For this, go in live view, switch to automatic focus (on the lens) and do a half press the shutter. One you hit the target, then go back to manual focus and you're done.