Just adding my two cents since some people seem to be misunderstanding:
when you find an Input Key and not a Spline Point it’s going to be a float, meaning it can be inbetween keys. At that point all you need is to call get distance at input key.
In my case:
auto InputKeyAppropximated = CameraSplineComponent->FindInputKeyClosestToWorldLocation(Character->GetActorLocation());
auto DistanceAlongSpline = CameraSplineComponent->GetDistanceAlongSplineAtSplineInputKey(InputKeyAppropximated);