leaflet - GTA V coordinates to LatLng

I'm here because I'm trying to create a GTA V map with Leafletjs API. In deed, I've got my tiles that are ready. But the aim is to put markers on the map by using GTA V in-game coordinates. To achieve it, I need to convert the coordinates to LatLong but after many researches, I can't find a solution.

At Stackoverflow, they advise me to ask my question here.

For now, I have a simple function that help me to try to "calibrate" my coordinates :

function GTAVConverter(x,y)
{
    var ex = 0.01255;
    var ey = -0.01260;


    var x = ex * x;
    var y = ey * y;

    var x = x - 486.97;
    var y = y + 408.9;

    return map.containerPointToLatLng(new L.Point(x+1440.45,y+127.45));
}

But I think there is a better way to achieve my aim. This function isn't really accurate, and if the browser window is resized, all the coordinates are moved.

Example that I want to convert :

I know that Leaflet API has a coordinates system but I don't know how it works.