Need some slant six help

[The Mad Scientist]

My Daily driver threw a timing chain.  I've got all that under controll.  However.  The timing cover is missing the timing tab/indicator.  It's been missing since I bought the truck, but now it's a bit of a problem... 
What I'm dealing with is an 85 model D100 pickup.  It's got hydraulic valve lifters and computer controlled (not electronic) ignition.  I've never worked on a slant this new.  The only location for the timing tab that I've ever been familliar with on the /6 was on top of the balancer and a bit to the left.  Were these just welded on?  Bolted on?  Were they omitted for computerized ign/carb?

I have a timing light with dial in advance.  Off memory I think I need 12 degrees BTDC.  The only thing I can think to do when I go to set the distributor is watch the marks on the balancer with the light at zero as I turn the dist. maximum both ways.  Find the point where the engine dies on both ends of the scale and mark those points with a sharpie.  Then measure and cut that in half and call that zero then set the advance on the light for 12 degrees.  Is there an easier way or something I don't know? 

Any suggestions will be appreciated

TMS

[tecmopar]

IIRC the '85's had the magnetic probe tube and tab located on the pass. side and was a bolt on piece so if its not there it may be a problem to get it just right. If there are any around, try a junk yard for it, can't be more than a few bucks, good luck.