Now I may be mistaken about this, but it seems to me that measuring to the top of the seat lug does not yield consistency. The distance from the centerline of the seat lug to the top varies with different lugs, does it not? The lug may extend ½ above the centerline of where the top tube intersects it or it might be ¾ above this point the fact seems to these jaded old eyes that this in no way changes the size of the bike. The upper point of the seat lug can be anywhere the lug designer wishes The size of the bike is actually determined by the intersecting points of the main triangle. These would be constant, where a possibly ornate and very long tanged lug could vary all over the place.
As to the standover height varying with the type of measuring, I have to wonder, why worry about it? It would be a very small variance. One just adjusts the angle of the lean of bike when mounting or dismounting. I guess any lean would be unacceptable to some, but when I see someone on a bike requiring no lean during a mount or dismount, it always looks like the bike is actually a bit too small for them. Nearly all of us learned to ride on oversize bikes anyway, it seems to me. This provided room for us to grow a bit without replacing the still quite good bike. We were very poor, so perhaps this is just a peculiarity of the lower economic class?
Getting very close to my daily limit of posts, so Id better vamoose,
Tom Sanders
Lansing, MI USA