I understand exactly Cazzex,,,Trust least whom claim to be most trust worthy... First point to make is....
"Too many cooks in the kitchen (spoil the broth)"... under the bonnet right hand side Brake reservoir has High and low level markers... Unless someone stupid enough has topped up the fluid it should give a decent indication of brake pad thickness...
Fluid close to full = brake pads near 80-90%
Fluid level low = Brake pads need replacing...
Now problem with European cars is... (depending on driving behaviour) is because the material of the disc Brakes are softer than American GM -H or similar.. Is the disc's need replacing every 2nd to 3rd time you replace the pads... to get them machined ("tolerance's") only creates another issue of heat buckling... Hint after along journey don't put the hand brake on if you can help it... Creates a heat sandwhich and will buckle the rear disc... GM-H holdens exempt Hand brakes inside the hub and not attached to calipers...
You can put a softer compound disc pad in you beemer to preserve Disc wear... However this will cause more ugly brake dust on front wheels and what you often save from softer pads you spend on more frequent changes...to brake pads...
