Converting world to local space with an entire transform is actually quite easy, IF you know how to.
The following example:
UnitTransform = the current actor that needs the relative location
InstanceTransform = the relative transform result
const FTransform UnitTransform = UnitPtr->GetActorTransform();
const FTransform InstanceTransform = UnitTransform.GetRelativeTransform(
this->GetActorTransform());