Every part of the base must be within for "wholly within", Page 3 of the Core Rules, Designer's Commentary
Q: Sometimes a rule will specify that a model or unit needs to be ‘wholly within’ a certain distance. What exactly does ‘wholly within’ mean?
A: A model is wholly within a certain distance if every part of its base is within the stated distance. A unit is wholly within a certain distance if every part of the bases of all of the models in the unit is within the stated distance. For example, a model would be wholly within 12" of the edge of the battlefield as long as every part of its base was 12" or less from the edge, while a unit would be wholly within 12" of the edge of the battlefield as long as every part of every base of the models from the unit were 12" or less from the edge.
https://www.warhammer-community.com/wp-content/uploads/2019/07/8f9bd00c.pdf
If it was the other way round where only a part of a single model needed to be within then large single model units could setup much further forwards during deployment, imagine the benefit of deploying a Dread Saurian this way.