Hi Lukefg,
I’m unable to pinpoint what the issue is here. Would you be able to please attach a couple of screenshots along with the model of the issue you’re having?
Depending on the way the material is estimated (whether it be via area, volume, etc.) will determine where to apply the material. Area will normally need to be applied to a face where as volume and lineal lengths will need to be applied to a component/group.
Would you be able to keep us posted on how you go?