DXM Dose Calculator is an invaluable reference tool for anyone interested in the recreational use of this primary ingredient in cough syrup (though abuse thereof is not encouraged by the content creators). With the imagined user's weight, desired plateau, and little more detail about how far into the plateau they want to go, this application provides the required dose in mg/kg, giving as well the total number of mg required for that dose, and an approximation of how many pills or liquid doses that may require.
As always if you are planning on recreating with controlled substances ensure that all safety precautions are taken - ideally: don't do it. In particular with DXM ensure that DXM HBr is the only primary ingredient. Anything else is ridiculously unsafe.