Timing help needed

I don't normally tune big blocks.
And the biggest cam I have tuned, in a 340/360, is the MP292/292/108, which IIRC is [email protected]..
That said;

I have found, that lots of Idle-Timing, by and large, covers a less than ideal Idle-AFR.
In my own combo, (Eddie-headed, 11/1Scr, 367/A833) I too started at 18* because it wouldn't run right otherwise. And the Idle-speed was up around 850 with an ancient Holley 750DP.
Eventually, after I learned a few things by trial an error,and I got the AFR right. After that, I applied what I had learned, and got that combo to idle down to 600/550 in first gear @5* advance.No I didn't run it there full time.
The rest of that summer and the next two even, I spent quite a few hours getting other guys ugly-combos running right.

The point being;
IMO,
if you get the AFR dialed in closer, you won't need a lot of timing.
Then;
Being as how your combo is an automatic, you can run more idle-timing than us Manual-trans guys, without incurring jumpiness, cuz most of that gets lost in the TC. And more initial timing also helps to get the cruise-timing, that your engine will crave. Also since your DCR is so low, you may not even experience jumpiness.
Then;
there is no good reason to run less than 16/18 idle-timing, unless it screws up your Idle-AFR,beyond what the cam is already doing, that is to say; destroys your Transfer slot sync.

To tune your idle, you have six tools in your tool-belt; namely;
1) temperature; of the Coolant, and Inlet Air Temp
2) the wet and running, fuel level,
3) the low-speed circuit; which consists of; the transfer slots, the emulsion tubes, the IABs, plus the mixture screws,
4) transfer slot synchronization,
5) idle-air bypass, and
6) Base-timing.

Your job as a tuner is to balance those cowboys on the AFR seesaw, to make the engine happy; not just at idle, but also as the throttle slowly tips in; to prevent sags,hesitations, or stumbles.

I don't know about big blocks,
but with STREET SBMs it is easy to get carried away with too much idle timing; especially with a, high-compression, manual-trans equipped, combo.
It's not that the timing is too much, rather it's a bad ratio of transfer slot, to mixture screw, to bypass air,etc., that calls for the extra timing, to make the engine happy.

You probably know all or most of this, but sometimes it just helps to see the options ... in print.

BTW; I don't have an AFR gauge, so I have to pay more attention to what the engine is telling me.