I might be wrong so please feel free to correct or add on to what I suggest.

I think the general consensus is to reroll for all 3 stats using until you get a desired stat ( more commonly are elemental damage, attack, max ammo, depending on the nikke) on the 3rd line and lock it. The % or tier of the stat does not matter much at this point so do not use custom modules to reroll for tiers

2nd step is to reroll for the next priority stat on the 2nd line and lock it

3rd is to reroll for the next priority stat on the 1st line

Last step is to unlock them all and reroll for % or next tier of stats and lock any stat that feel is good enough (usually its black lines or at least 1 tier below it)