To tune my auto trans car's Holley I warmed it up and put it in D with the E-brake on and tuned for a mix of best vacuum and idle quality. A stock cam will like 14:1 afr for idle, a wilder cam will want a richer mix all the way down to the high 12s because of reversion.

Don't tune the idle in N tune it in D. If it has to be 1500rpm in N just to idle at 750rpm in D look for more initial timing or a higher stall converter.