Need help for snapping to Grid

Hey Guys, I am making an RTS game and I am trying to snap my Ghost prefab building during run-time after I instantiate it using the UI button, and I want to use the Mathf Round function. However, for past month, I have tried many functions to get it to work, including the Mathf Floor, but the Ghost prefab does not snap to grid for some reason, so I removed the function. Here is my script, if anyone can help that would be nice. Thank you.

Code (CSharp):

1. using System.Collections;
2. using System.Collections.Generic;
3. using UnityEngine;
4. using UnityEngine.UI;
5. using System.Linq;
6.  
7. public class ObjectPlacement : MonoBehaviour {
8.    
9.     public GameObject BuildingPrefab; //Actual building. This will be placed from another script called 'BuildingSafe'.
10.     public GameObject GhostBuildingPrefab; //Ghost prefab which will be instantiated after clicking the UI button.
11.     private Transform currentBuilding;
12.     public float  RotateSelf = 3f; // for Rotating the building during runtime before placement.
13.     private Transform transform;
14.     private BuildingSafe safeBuilding;
15.  
16.     // Use this for initialization
17.     void Start () {
18.     }
19.    
20.     // Update is called once per frame
21.     void Update () {
22.  
23.         Ray ray = Camera.main.ScreenPointToRay (new Vector3(Input.mousePosition.x, Input.mousePosition.y, Input.mousePosition.z));
24.         RaycastHit hit = new RaycastHit();
25.         if (Physics.Raycast (ray, out hit, Mathf.Infinity)) {
26.             hit.point = new Vector3(hit.point.x, hit.point.y, hit.point.z);
27.             currentBuilding.transform.position = hit.point;
28.         }
29.  
30.         if (Input.GetKeyDown(KeyCode.R)){
31.  
32.             currentBuilding.transform.Rotate(Vector3.back, RotateSelf, Space.Self);
33.         }
34.  
35.         if (Input.GetKeyDown(KeyCode.Q)){
36.  
37.             currentBuilding.transform.Rotate(Vector3.forward, RotateSelf, Space.Self);
38.         }
39.  
40.  
41.     }
42.  
43.     //For instantiating a Ghost prefab after clicking the UI button.
44.     public void CreateBuilding() {
45.         currentBuilding = GameObject.Instantiate(GhostBuildingPrefab).transform;
46.         safeBuilding = currentBuilding.GetComponent<BuildingSafe> (); //The Ghost prefab will check whether the building is safe to place or not before clicking.
47.         safeBuilding.BuildingPrefab = BuildingPrefab;
48.     }
49. }