Modern digital screen visual acuity projectors are more accurate than old school bulb projectors. However, calibration of the projectors is similar. With digital screen projectors, you still need to measure that distance from the chair to the chart. You will need to enter this into the computer. There is also a calibration square that measures 10 cm by 10 cm. You need to actually get out a ruler and make sure this is accurate. If it isn't, there is an adjustment on the screen that can be done. A video going over this can be found here.
The projector tube of an old school bulb projector can be adjusted so that the projectors letters change their size. There is a chart provided with the projector that shows what the size of a 20/200 E will be at various distances. For instance, at a chart to chair distance of 20 feet, the big E should be about 87.24 mm. If the chart to chair distance is 19 feet, the big E should only be about 82.88 mm in height. So, you measure the distance from the exam chair to the screen. Take out your calibration chart that goes over what the size of the letters should be at your particular distance. You then turn on the projector and set it at 20/200 with the big E. You go over to the wall and measure the height of the big E with a ruler. If the big E is not the proper size according to your chart, you adjust the projector tube so that the size of the projected letter matches the size in the chart.
A video going over this can be found here.