how can i get y (hight) of Navmesh

i want a frog to move randomly on the Navmesh.
so i have a random Vector3 in a Sphare radius (around the frog)
i take the Vector3.Y and i move it up 800f (to make sure that its not under ground)
then i shoot a ray down 1,000f in length.(to make sure im passing the ground)
now if im hitting the ground im setting the NavMeshAgent to hit.point

im trading to make a game without physics so i cant use Raycast.
correctly im casting a ray to determine the y of the floor.

i have Z and X axis.
can i find the y in a different way? (without physics )
can i get the correct Y on the navmesh surface?

Code (CSharp):

1.     private void JumpAround()
2.     {
3.  
4.         Vector3 pointInSphare = Random.insideUnitSphere * _sphareRad;
5.         pointInSphare.y = 800;
6.  
7.  
8.         Ray ray = new Ray(pointInSphare, Vector3.down);
9.         RaycastHit hit;
10.  
11.         if(Physics.Raycast(ray ,out hit, 1000.0f,  Layer))
12.         {
13.             pointInSphare = hit.point;
14.             _thisNavMeshAgent.SetDestination(pointInSphare);
15.         }
16.     }