i just read your other post.
so it seems that the engine has had the chain guides replaced at some recent time. so there is no base from which to work.
let's now agree that to the right of 0l0 is btdc and to the left is atdc. but before we go forward i'm going to suggest you resolve the tapping sound first. i'm just getting my 88 back up due to a tiny tapping sound which ended up costing a new cam and set of rockers and i valve compensator. if the valve comp is bad easy fix as long as no damage has occurred to the cam or rocker. so do that first. (note: if the face of the cam and rocker are bad it is likely do to a lack of oil which means you need to replace the oil tube plastic and the inside of the tube needs to be cleaned. these valve train parts seem to be surface treated for hardness. my cam is cast iron because you can tell by the rough surface area. the rocker looks like a steel forging.
once the ticking is repaired you can move on to the timing chain.
you also said you have had the car for just a short time so here is
a few things to look for:
make sure the notch on the right hand (pass) bank is in line with the mark on the cam bearing tower.
now check the left bank and compare they should be close and in the same sector meaning both should come up to their marks about the same time. if the left is already past it's mark as the right is coming up to it's mark then you may have a mis aligned chain....but you said it runs good so likely you won't see this.
the next step would be to look at the woodruff keys. are they offset or straight. now say they were as much as 10deg offset and the new chain installed it is possible your timing could be advanced past 0l0 and would show Btdc.
now in my head i'm thinking that if you have straight keys (not offset) maybe just maybe who ever did the work could have jumped a tooth and gone for the 18deg advancement you get when you jump a tooth. (just like using offset keys except you get 18 deg for 1 tooth.) you see now your description of being btdc would start to make some sense to me. other guys can correct me if i'm wrong but even a new chain has no guarantee of bringing the timing marks back to 0l0 (tdc) which is basically what we are trying to do.
as far as working ok or functioning properly i think it will. ( you've already said it works ok now) ignition timing is relative. you pull the vac tube or not then you turn the dist clockwise to advance ccw to retard. (i have a 72 350sl which is just like your 73 450sl and i will be working on it from the weekend on) i understand my 87 & 88 doesn't work that so turning the dist does nothing. but these have points and dwell etc so i'm thinking they should work similar to us ignition timing.
so with all this said it would seem to me that you'd adjust the dist till you acheive your idle setting of ??. then as you raise the rpm you can physically see your total advance timing probably some where of 30 deg or so.
but make no mistake about the tapping if it is in the valve train you need to address that first. cause who wants to go back in there in a month or so.
i would think that once you've figured out how your engine is set up it will all fall into place of how it works. you compression looks tolerable so no apparent reason for it to not run good.
get yourself a sprial bound note book like 5 x 7 and document every thing you do no matter how trivial. like your compression oil change dates gas mileage all the above info you're going to get after checking the timing out.
good luck and keep us posted
1972 350sl Red/Blk 117k
1988 420sel charcoal/Blk 140k
1987 420sel gold/tan 128k
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