ICAO Annex 18 woud be a place to start.
Give yourself about 2 days to read it and another month to get your head around it.
Also sometimes the IATA DG Rules are of assistance. Difference between the two being ICAO covers the carriage and IATA the shipping. Then throw in the extra's of your local NAA.
I remember slinging charges in Myanmar on a seismic job.
We were using B206L3's and the ICAO DG instructions came up with a limit which was about half the available payload of the aircraft and we were slinging the stuff on a long line. The amount on the sling would have vaporised the aircraft as it was so it would have made no difference to load the aircraft up!