Advance the timing before TDC. Why?
Why do I want it tuned that way?
as to WOT Timing
Because there is a time (in degrees ATDC) in the power cycle when the crank is in the best position to receive the energy of the hot expanding gases atop the piston. If yur too early or too late, the engine cannot make power to it's potential. I have read variously that this point in time is from 25 to 28 degrees ATDC. So this is, or should be, what your timing is designed to do.
As to Part Throttle Timing,
It is critical to get your sub-3600rpm timing to that ideal point, for the engine to make the most amount of power it can, with the least amount of throttle. This can take over 50 degrees of lead to achieve.
As to Idle-timing
At idle your engine might like 20 to 25degrees of timing. I mean it is easy to figure out, just keep adding timing while simultaneously maintaining your idle-rpm by reducing your throttle opening, until the rpm no longer rises with more timing.
But just because the engine likes it, doesn't mean you should give it.
The reason being is that as the throttle is reduced, less and less fuel will be delivered from the Transfer slots, so more and more will have to come from the mixture screws. Ok fine, until the mixture screws get maxed out. And then, it won't take throttle because the transfers are either dried up, or too slow to get moving again, and this results in a tip-in sag, a hesitation, or a bog. Sometimes you can cover that with pump shot, but there is no good reason for that, except to satisfy the tuner's personal desire to throw mega idle-timing at it. Tuned this way, she will be a pig on gas, and possibly prone to detonating when the Vcan comes on line..
Your engine does not care
about low-speed timing, until it hits it's stall rpm, and again at power-timing, usually after 3200 to 3600 rpm. Below stall, with an automatic, it doesn't much care about the timing, as long as it is close.
However, the mechanical timing is designed for WOT operation and can only be made ideal at three points; the rest of the time, at anything but WOT,
the mechanical timing will be 100% wrong! The three points are as mentioned, stall rpm, all-in-by, rpm (usually 3200 to 3600), and whatever it takes at idle to not have a tip-in sag, or too high an idle or if it bangs or stalls, going into gear.
At all other times when NOT at WOT, your Vcan has to make up the difference from what it's getting, to what it wants.