If you're looking for the equivalent of 30mg insufflated then probably around 15-20mg would be sufficient. If you're looking to hole then somewhere around 40mg would be (imo) a good place to start, although if you haven't tried higher dosages then I'd suggest trying out higher dosages insufflated first to get a feel for it without being thrown in the deep end
^ MXE mixes really nicely with opiates, 10-15mg MXE plugged really adds nicely to the opiate high. Any more tends to overpower the opiate but it's still a great combo, very jealous as I'd love to try MXE and morphine :D