Depends by the brand and type of the atoms.
For example.
With the 510, for 6v, I use 3 ohm with Cisco. With the IKV 2.5 ohms. With Joye, SLB, EM... also 2.2-2.3 ohms to 2,5ohm, because I noticed how each brand has a different performance (so a 3ohm on a cisco has, more or less, the same hit a...