A lot depends on the juice and airflow of each device, but I've found that higher voltages don't work so well for cartos. My main vape are Boge F15 LR with an extra hole punched, loaded in IBTanks on top of a Vamo. The current carto is reading 2.2Ohms and my wattage is set for 6.5 (around 3.8 Ohms if my maths is right). Loads of vapour and taste, and the cartos are currently lasting on average about five days before I start getting a burnt taste or leakage. The juices I use don't seem to like high voltages anyway. It's very noticable if I get the slightest hint of scorching from the carto.