I think it may be easier in the automatic than the manual transmissions but it works the same way.
I can't remember off hand how the manual dust shield is set up. It's been a long time since I've seen one. I thougt they had a dust shield that was removable. Even if it won't come out completely you should be able to get enough clearance to see up in there.
The shield on the automatics is easy to remove. You have to remove it to change the starter anyway.
I simplified the drawing but you can do it from below... It's just awkward. If you can't do it with a drill bit then any 1/8-inch wire that will reach will work. You can bend the wire if needed.
You can do it thru trial and error but there is a risk of breaking the starter or worse the flywheel. You don't want to make a big change but a small one is usually safe.
You could try adding one shim but I don't think I'd do more without using the gauge, especially since you've not done many starters.
Most of the time you change shimming, it's a small change. Usually One whole shim in or out. I wouldn't mess with half shims unless you can gauge the setup.
They are hardly ever perfect but the closer you can get to specification the better for the starter and the flywheel.