It depends on how you build out your Merlin. If you go single coil with the air reducer in to simulate a more restrictive mtl feel and a resistance >1 ohm, you can likely use the same juice. If you decide to build a 0.4 ohm coil with full air flow, you might be better off with 6 mg. Only way to know is build it and test it.
For example, right now I use 12 mg nic for my Merlin and Bellus tanks, both ~0.5 ohms. My Crius Plus at 0.3 ohms has 6 mg in it (12 was too much). However, for my Limitless+ with a 0.5 ohm build, 3 mg is enough with the amount of vapor that tank can put out. So it isn't just a matter of just being "subohm" but very much about how it all comes together, especially vapor and airflow.