For the charuco markers themselves, I think you need a 1 “pixel” border, pixel in this context meaning the size of the black/white squares within the Aruco marker. So you can probably get a lot closer to the corners if you want…by generating the Charuco target at higher resolution than your monitor can display, then cropping it to take of, say, 50 pixels left/right/top/bottom. As long as there is enough white around the Aruoco markers, the rest should work. (I think) Also you can probably get away with slightly larger Aruco markers if you are having trouble locating them.
Another thing…if it hasn’t come up already. You want to be displaying your calibration target without any scaling / resampling if at all possible. You want each image pixel in the calibration target to correspond to exactly one pixel on the monitor for best calibration results.