I just recently purchased a 98-6 level myself, for my new PM 1236 lathe, and PM 962 Mill.
Using the level has been a great experience, and I have come up with all sorts of uses for it on the Mill, probably known and written about by others, I just have not been able to find anything similar to what I am doing.
Before I go on, the 98-6 was not perfectly flat on the bottom, I could press on one corner sharply and it would tap the plate. About .0002 clearance, and would shift the bubble a bit if you held it down. I took a permanent marker and marked up the bottom, rubbed on the mill table, and the lifted corner was barely touched, and the others about a half inch was cleaned off, though diagonally from the lifted corner it was rubbed off about 3/4s of an inch.
I Called Starrett, and Scott the technician educated me a bit on levels. Machinist level bottoms are slightly bowed, so you have to have a surface longer than the level to place them on, as only the ends touch the surface you are leveling. That was good to know !
Mine was definitely warped, and he said return it. I do not get Starrett, no instruction in the box, no instructions online. I find that odd.
I am still not positive how much to tighten the adjustment screw and nuts. I forgot to ask the technician while I had him on the phone
I have been loosening both nuts, snugging the screw, the adjusting the bottom nut first when matching the bubble, then snugging the top nut slightly, rechecking, and then snugging up just a bit more on the nut that will bring the bubble into the perfect position both sides. Then check again. I do this on a rigid precision flat surface on the mill table, in exactly the same position when reversing. And I give half a minute between adjustments for the bubble to truly settle. I do not breath on it, I do not have hot lamps shining on it, and I make sure the room has been at a steady temperature for many hours.
Anyway, I am thinking that it is not important that the mill table be level in an absolute sense. The level can give a quick check of your table one position relative to another and will reveal any twists, depressions or bows. You will need precision bars to put the level on, that bar length can be whatever you want it to be, depending on what you are checking for. A 6 inch level should be the only level any hobbyist would need ?
The most trick use is to compare the table to the vise. I have not come across any posts that discuss calibrating the vise to the table. It can be done a couple of different ways with a dial test indicator, but the level is quick and easy. I lay the level on the same surface you place the parallels on with the level on the same bar and on the same spot on the bar as when it was on the table., if the bubble is at the same spot as it is on the table, bingo !
This level is so sensitive, that is why it needs to be used on the same bar on the same spot throughout. And they make one even more sensitive !
I am sure if you buy a Kurt or Glacern vise, it is perfectly square to a high precision, but what about used or imported vises ! Assuming the surface the parallels sit on is parallel exactly with the table would be a mistake I am thinking.
I am thinking that lathe owners trying to level their lathes may be laying the level directly on the ways, and this will give a reading that represents the relative difference between the small areas at the ends of the level. I really think the only way to use a level for anything is to lay in on various length precision bars, with the level on the same position on the bar.
But then again, I am not so positive about anything I just wrote, I just noodled it out the other day !
RH