How do I convert odom and pose information to grid location on occupancygrid map [closed]

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I am also using the occupancygrid.msgs type found in the topic /map to get the costmap information found in member data. I am trying to figure out how to convert odometry and pose type data(x,y) to grid index[x,y] if that makes sense(this is after I convert msg.data to a 2D array that represents the 2D map). I have tried using the resolution found in the .yaml file of created maps but that doesnt seem to work very well, unless I am getting the calculations wrong? If there is any confusion to my question please inform me so I may try to explain more.

The calculations I use to convert pose to pixel are here below(please not below is not actual code, its just i code form to delinate it from the rest of the text as calculation)

origin:[-10,-10,0] which are the x,y and yaw respectively
resolution: 0.05 meter/pixel

to convert poses to  [i,j]( pixel location/grid index)
i= ("pose.x" - (-10))/0.05
j= ("pose.y"-(-10))/0.05

It doesnt work completely accurately for some reason. For context, I made an algorithm that generates points that I use as goals for the robot to move to. some times , some of the goals are outside of free space of the map(that is to say the some goals appear in physical co-ordinate points whose cost value in the occupancy grid is either -1(unknown) or 100(occupied), What I wish my algorithm to do with these points is to completely skip them and not assign them as goals. Here is some pseudocode to explain what I thought would be an absolute solution:

import rospy
from nav_msgs.msg import OccupancyGrid
def getcell(msg):
    global grid
    h = np.array(msg.data)
    #grid = h.reshape(384,384)
    grid = h

rospy.init_node("controller_3")
rospy.Subscriber("/map",OccupancyGrid,getcell)

.........
ex = ((goal.pose.position.x-(-10))/0.05)
ey = ((goal.pose.position.y-(-10))/0.05)

if grid[math.ceil(ex*384 + ey)] == -1 or  grid[math.ceil(ex*384 + ey)] == 100:
    Skip the points

Points are still being created in grid cells that should be either -1 or 100 in cost value.