318 mystery problem, somewhat solved!

I doubt it will ping at idle no matter what you do (within reason).

With a reasonable ignition system and our slightly archaic combustion chamber designs, 17:1 is probably about where you'll start to run rough due to lean misfire. 15:1 is pretty close to stoichiometric and should give you a steady "clean" idle. That is, CO and HC emissions will be minimized and enough fuel will be available for smooth operation.

I honestly don't know a lot about the FAST systems or which one you're running My experience is mostly Bosch Jetronic (eurocar) and Rochester TBI (simple, hackable, effective, CHEAP). But I'll pass on a couple tuning thoughts I have...

Idle tuning: Your computer GUI should give you a MAP readout and possibly an IAC % readout. Since the computer adjusts the IAC to maintain a specified idle RPM it's a little harder to set it like we used to with carbs. In the old days we would adjust the mixture screws for either highest vacuum and/or smoothest idle. You can unplug the IAC motor and still do this (you've disabled an electric screwdriver on the throttle plate screws) or, you can make changes and see how it changes IAC open %. If you unplug the IAC motor start a bit rich like 13:1, unplug, and lean in steps until RPM or MAP peaks and begins to drop (50 RPM drop from peak was considered "lean best idle" when these cars were new, and works well now). If you can monitor IAC open % lean until the IAC motor begins to open and call it good. Either way you're basically homing in on the engines "sweet spot" much like old school vacuum gauge and by ear tuning.

Acceleration: Best power and detonation resistance is around 13:1. Anything much above this is wasting fuel and shortening ring life. Don't worry about leaning out under sudden acceleration. There should be an "acceleration enrichment" or some such algorithm in the software. This is basically a throttle pump. It recognizes rapid TPS opening and momentarily biases the mixture richer. Monkey with this last, just like pump shot, generally less is more.

Cruise: Much like idle, you're looking for the sweet spot. You can't look at IAC % so you need to look at MAP vs. RPM. Start somewhat rich, like 14:1 and lean to peak. You should end up at 16-17:1. Keep in mind that a leaner mixture likes more timing advance, so adding advance to higher MAP tables can be of great benefit. This is what a vacuum advance does. If you have spark control, you should be able to bias a table for this.

Reality check: my old totally stock 72 318 Dart (PS, PB, AC, AT) got 18-22 MPG. As I modded it with Carter 9636, LD4B, 340 cam and 2 1/4 duals I always maintained this. This should be your goal, and it's easily achievable.