I do not agree that this way is working (in case of medium or small format cameras) ; when I have to check the focusing, I do it systematically by test shoots with real film (an old wife practice, I guess).You can check infinity focus by aiming at an infinity target, setting the lens at infinity and checking focus at the film plane. A bit of Scotch frosted sticky tape stretched across the rollers makes a good temporary 'ground glass'.
For two reasons :
1/ usually, accurate focusing means at least 0.03mm detail resolution on the neg (maybe even smaller on small format). So, are you sure that you can see this - naked eyed - on a makeshift groundglass ?
2/ when you test for focus accuracy, you should do it lens full open ; let's say, for a MF camera it means f/stop 3.5 . To reach focusing details of less than 0.03mm, you are at this f/stop not allowed a focus plane misplacement error of more than 0.1mm : are you sure that your "bit of Scotch frosted sticky tape stretched across the rollers" is positionned at the same place as a film with an accuracy better than 0.1mm ?