For diffs "0" lash is ideal.. pinion (input) bearings yield quickly and will ease off on the mesh increasing lash.. the closer you can get to "0" increases your chance of ring and pinion longevity.... zero lash is not to tight in these cases due to bearing yield..
Start with 1) .2 shim and add whatever it takes to achieve no backlash..
It's the easiest way to explain it..
Before any of this is even done your input bearings (assuming they are good or new) should be preloaded...
Loosen the input cup grub screw, use a thumb and pointer finger to compress the cup onto the pinion (input) shaft... With moderate pressure on the input assembly, tighten the grun screw.. this will preload your input bearings, allowing no play in the input at all.. a wiggle test on the input cup will confirm your preloaded...
If you would like to take it a step further you can increase heal to toe tooth engagement by shimming the pinion, either behind the large internal bearing or behind the gear itself.. the preferred method would be .2 behind the bearing...
If you also choose the latter method, start from scratch with no shims on the diff carrier stack, and work your way in as mentioned above...
Hope this helps and doesn't send you further down a rabbit hole...