If the level is facing in exactly the same direction, the drill bit didn't move and is exactly the same distance from the end of the level when it is flipped around.
My only objection to this is thatdon't bother with the drill bit.
set the level down. Note where the bubble sets. spin it end for end setting it in the same place. Note where the bubble sets.
rotate top for bottom. note where bubble sets. (setting back in the same place)
spin end for end. note where bubble sets.( set in same place)
If the bubble sets in the same relative position to the line on one side or the other of the bubble (the same side of the bubble each time from the current perspective) the level is accurate.
You can do the same for the plumb bubble.
unless you have microscopic eyes and the bubble is exactly the same width as the distance between the lines, there is always error. If the bubble does not fill the line gap exactly, you will always be guessing how far out of level the level is setting. is the bubble 1/16" from the line; 1/32"; 1/64"; etc.?And with the drill bits you can measure how close you are to the 500 microinch per inch Grainger spec'n.
That's my point. I think:unless you have microscopic eyes and the bubble is exactly the same width as the distance between the lines, there is always error. If the bubble does not fill the line gap exactly, you will always be guessing how far out of level the level is setting. is the bubble 1/16" from the line; 1/32"; 1/64"; etc.?
Obviously one would be best served by utilizing as level a surface as is available for my tests but the comparison tests will prove the level against itself and unless you have a certified level to check it against, that is as close to checking the accuracy as you can get.
If the bubble locations do not match, there is some part of the level that is not true with the others. Now, if you want to find out where it is, be my guest but at that point, I toss the level. It will never be dependable unless you were to have it machined and calibrated. Not worth the cost unless it is a very large level.
A benchmark. Yes.Why not just nail one end of a piece of 1X to the wall put the level on it ,raise the 1X until level and nail the other end. now you can flip the level as many times as you want and even check against other levels.