I agree - maybe there should be another "global" setting - probably in the aircraft setting page, giving the default cargo type mix for each kind of aircraft. Already for most of my routes I am selecting 65/35 or 70/30 rather than the default 60/40. One issue I can think of with that is - it would be fine for now while cargo demand is linked to pax demand but in future, I imagine a default setting to be of little use as (hopefully) each route will vary more dramatically - some will have mostly heavy or medium, some only light, for example.
Also, can I note here that there is a minor bug with the cargo-mix slider - it's probably not even worth opening a bug report. The percentage numbers at each end of the slider don't update properly until you let go of the mouse button. So, If you move the slider to, for example, 80/20 and then let go, the figures change to 85/15 and you have to move it again. So you have to anticipate what the values will be rather than looking at the numbers. Sorry if that isn't very clear - try it and you'll see what I mean.