In conjunction with current density, you have to consider anode exposure.
Electricity will ALWAYS take the path of least resistance, so parts buried within a pile will not get much current! Parts on the outer edge of the pile, directly exposed to the anode, will get the lions share of the current.
So a "shadow fudge factor" should be applied. Unfortunately, this is not a cut-and-dried number, you just have to eyeball your pile, and guess at the percentage of parts buried in the middle of it. Then reduce your total current by that percentage. And this will be different for small vs. large piles of parts.
eg. I normally run at 80 ma/sq-in (old formula electrolyte) with wire-hung parts. In barrel runs, I use 50-60 ma/sq-in, since only the outer parts are being plated.