Convert Latitude Longitude To Correct X,Y Values

Hello Experts,
I’m developing an application with Three.js and I would like to see correct position on the map using with latitude longitude values. I got the static image from the mapbox which has 1280x800 resolution and 15 zoom level. I’m using the following code but it doesn’t work that I expected. Where am I doing wrong ?
Here is my part of code and screenshot image:

function setupMapLayer() {
new THREE.TextureLoader().load("map.png", function (oTexture) {
    oTexture.encoding = THREE.sRGBEncoding;
    var oFrontSideGeometry = new THREE.PlaneGeometry(1280, 800);
    var oMaterialFrontSide = new THREE.MeshBasicMaterial({ map: oTexture, side: THREE.DoubleSide });
    var oFlagFrontMesh = new THREE.Mesh(oFrontSideGeometry, oMaterialFrontSide);;
    var oFlagObject = new THREE.Object3D();
    oFlagObject.position.set(0, 0, 0);
    oCamera.position.z = 500;

function setupMarker() {
var oGeometry = new THREE.SphereGeometry(10, 100, 100);
var oMaterial = new THREE.MeshBasicMaterial({ color: 0xFF0000 });
var oSphere = new THREE.Mesh(oGeometry, oMaterial);

let oLatitude = 40.8746966; //North-South
let oLongitude = 29.3988391; //East-West

//latitude goes between -90 degrees and +90 degrees
let normalizedLatitude = 2.0 * (((oLatitude + 90.0) / 180.0) - 0.5);
//Longitude goes between -180 and +180 degrees
let normalizedLongitude = 2.0 * (((oLongitude + 180.0) / 360.0) - 0.5);

//Our bounds along the x-axis are -200, to 200
//and along the x-axis are -100 to 100.
let oXLocation = normalizedLongitude * 640;
let oYLocation = normalizedLatitude * 400;
oSphere.position.set(oXLocation, oYLocation, 10);

Try it with this formula:

Hi Mugen,
Thanks for your help. But when I applied this formula the circle always stays same position. Do I need to minus these coordinates values from the two dimentional map’s center position (0,0) ? In addition to I wonder this coordinate values returned from calcPosFromLatLonRad function is meter or milimeter ? I don’t know exactly what I need to do. Here is the result image when I applied that function. Any idea with this situaton ?

I’m also tried two different map image. But same coordinates seems different positions. The image that I used is using web mercator projection(I got the image Mapbox) the other one is using equirectangular projection. I think the problem is occurs with this situations.
Here is the image link: