Thanks for the input, Roadrunner. You're right as rain about the "proper" setting of the ride heigth.
On this 2001 Magna, the front setting (8 5/8 inches bag height), is duck soup to measure and set. Not so with the rear. With two bags on each side (I'll call them leading and trailing), it is very difficult to determine if the ride heigth is set properly, which is,,"8 inches, measured from the bottom of the frame rail to the bottom of the bag" (i.e. the support plate for the bag).
Yeah,,right! Measured at what point? Front of the leading bag? That's easy. At the rear of the trailing bag? A little more difficult (for a fat person). You can forget measuring in-between the two bags! Take a look.
The reason I point this out is, there will be a big difference (1 inch) between these two measurment points, regardless of the present "setting". The rear of the trailing bag being the shortest. Stands to reason, what with the engine/trans hanging out back there.
A "common" point, such as you mentioned ( axle center up to bottom of wheel well faring) would be great but, are all wheel well openings created equal? Can you hang your hat on that?
Prior to this Magna, I had a 37' Vogue II (1984). It had 8 airbags ( 4 & 4). Same heigth control valve setup (3). Again, the front was easy. The rear setting did not concern itself with actual bag height but, as you said, a measurment from the center of the axle up to an easy referance point on the coach. Duck soup!