Spawn prefabs in a vertical stack with a fixed gap

I am making a simple game with a ball that moves left and right when it hits a wall. I want the game to be like an arcade type game where the ball is on a platform and it keeps jumping up.

I've written code to spawn a list of prefabs randomly stacked vertically with a fixed gap. Since this is an endless runner, I want the bottom Prefabs to be destroyed and new one to be spawned on top of the previous one.

But when a new prefab is being spawned, sometimes the gap made on the y axis is off by a decimal point, and the more you play, the bigger the gap is, until the ball can't make the jump to the next platform.

Any Solutions?

using System.Collections;
using System.Collections.Generic;
using Unity.VisualScripting;
using UnityEngine;

public class PlatformSpawner : MonoBehaviour
{
    public GameObject[] prefabs;
    private GameObject platform;
    public float gap = 3f; 
    public int maxInstances = 20; 
    
    private int numInstances = 0;

    public ArcadeMovement scoreCountScript;

    private float spawnerPosition = 0f;
    private float lastSpawnedPosition = 0f;

    private GameObject newPrefab;

    private void Start()
    {
        spawnerPosition = transform.position.y;
    }
    void Update()
    {
        float scoreCounter = scoreCountScript.scoreCount;

        Vector3 currentPosition = transform.position;
        Transform currentRotation = gameObject.transform;
        currentPosition.x = 0f;
        currentRotation.eulerAngles = new Vector3(currentRotation.eulerAngles.x, currentRotation.eulerAngles.y, 0f);
        transform.position = currentPosition;

        if (numInstances < maxInstances)
        {
            int index = 0;
            if (scoreCounter < 25)
            {
                index = Random.Range(0, 3);
            }

            else if (scoreCounter >= 25 && scoreCounter < 50)
            {
                index = Random.Range(0, 5);
            }

            else if (scoreCounter >= 50)
            {
                index = Random.Range(0, 7);
            }

            currentPosition = transform.position + new Vector3(0f, Mathf.RoundToInt(numInstances * gap), 0f);
            newPrefab = Instantiate(prefabs[index], currentPosition, Quaternion.identity);
            currentPosition += newPrefab.transform.position;
            numInstances++;
        }
    }

    private void OnTriggerEnter2D(Collider2D collision)
    {
        if (collision.gameObject.name.StartsWith("Platform"))
        {
            Destroy(collision.gameObject);
            Debug.Log("Platform Destroyed");
            numInstances--;
        }
    }
}