Yes, in the front it's the vertical distance between the inboard and outboard pivot pins on the lower wishbone (AKA lower control arm -- interchangeable terms), and like all of this stuff seems much simpler after you get your head wrapped around it -- if ever. [Like algebra. ;o) ] Diagrams and numerical specs in the BBB and TDM. M-B had a jig/tool for measuring it; our Arthur Dalton, too. Or you could measure them both up from a common surface, such as the one the car is setting on, and compare for the difference.
And the rear height is determined by the camber, specified at: 1 degree positive +/- 30'. That is, the wheel tilted at an angle of 1-to-2 degrees from vertical, top outward. Measure angle from vertical with A. Dalton's specialized homemade $5 plumb bob tool or a $5 magnetic angle-finding tool (also plumb-principal) from Harbor Freight (not in Auto Tools: over with the carpenter's levels and rulers).
M-B's 1969 Technical Data manual, Section 40: Wheels and Wheel Alignment; Load for Measuring Vehicle, specifies that measurements for alignment issues be done with the car in "curb condition": includes oil and water, full tank of fuel, spare wheel and tool kit. There is a further standard loading termed "Test Load", which specifies 75 kg on each seat and 40 kg in the luggage compartment. [Gets nicely around the "trunk"/"boot" issue, doesn't it?] These pages further mention ambulances, funeral cars, heavy police radios and taxi partitions but I haven't seen the reference to our steel tops that other members apparently have found, likely in other editions of the TDM. [My wife says I'm hopeless at finding things -- sound familiar?]
So everything starts from the differential in height of the lower wishbone axes at the front, and with the wheel camber at the rear; get them right and you get the ride height your're meant to have - - excepting only the differences in tires, which can be considerable. And, since both the front and rear specs are given as ranges, the extremes of which appear to result in ride height difference of more than an inch, there's a range within which you can be 'right'.
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As a sort of PS, somewhere in all the citations resulting from Peter's Ride Height link a few posts above, there is a post from our MBZSE quoting an earlier posting by SD280SL, who cited the "M-B Book of Tables, 1969 ed." to the effect that 113 vehicle ride height should measure "160mm nominal" (~ 5 11/16) from ground to floor pan in the area of the structure directly below the door handles. [I remember one of these gents as "Hans in Sweden" in a signature block but can't sort which is which in my memory. And I think SD280SL is N.A. - no longer amongst us.] This is interesting and might be a useful benchmark. I haven't found it in my '69 TDM nor tried it on a real life 113 yet, but at nearly six inches it seems reasonable. Meantime, those taking spindle(or star)-to-arch measurements might check the 160mm metric to see how it correlates with their 'S<->A' measurements. [Thanks to Jim Villers for that very useful metric.]
