You have it right.

You hold the crank steady with the ratchet (clockwise), and loosen the cam bolt with a box-end wrench (counterclockwise). The trick is to get everything positioned so that you can push on the crank and pull on the cam at the same time. You reverse the motion to put it back together.

Needless to say, an assisitant is a great benefit.

Another way, at least on the 380, is to stick a bolt through the hole on the cam sprocket, and lodge it against the cam tower to hold the cam, but it's hard to do that and keep the marks aligned.

The official way is to use a special tool that fits over a cam lobe to keep the cam from turning.

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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe