OpenLayers with Custom Map Tiles and Bounds - Bound and Rendering Issue

I am using a custom map image. And I have verified my lat, lon bounds via google maps. But when using the code below combined with my image - it sometimes returns a negative number for y based on the different zoom levels - so I end up with an image path of (tile_z_x--y.png)

I am not really sure why this happens. My image is a rectangle - it is taller than it is wide, but I don't see how this would make a difference.

when I log the error - the max number that is returned for bounds.top is 29.1427626 but this is way outside my real bounds.

 function loadmap() {

    var left = -81.5426684;
    var top = 29.0901949;
    var right = -81.4901007; 
    var bottom = 29.1393509;

    var centerLon = (left + right) / 2;
    var centerLat = (top + bottom) / 2;


    var get_my_url = function (bounds) {
        var res = this.map.getResolution();
        console.warn(this.maxExtent.top);
        console.warn(bounds.top);
        var x = Math.round((bounds.left - this.maxExtent.left) / (res * this.tileSize.w));
        var y = Math.round((this.maxExtent.top - bounds.top) / (res * this.tileSize.h));
        var z = this.map.getZoom();

        var path = "tile_" + z + "_" + x + "-" + y + "." + this.type;
        var url = this.url;
        if (url instanceof Array) {
            url = this.selectUrl(path, url);
        }
        return url + path;
    }

    var options = {
        maxExtent: new OpenLayers.Bounds(-81.5426684, 29.0901949, -81.4901007, 29.1393509),
        numZoomLevels: 5,

    };

    var map = new OpenLayers.Map(
    'map',
    options

    );

    var tms = new OpenLayers.Layer.TMS(
    'Aerial',
    '/maps/output/',
    {
        type: 'png',
        getURL: get_my_url
    });

    //add a point
    var vector = new OpenLayers.Layer.Vector("Polygon Layer");
    var point = new OpenLayers.Geometry.Point( centerLon, centerLat);
    var pointFeature = new OpenLayers.Feature.Vector(point, null, null);
    vector.addFeatures(pointFeature);

    map.addLayers([tms,vector]);
    map.setCenter([centerLon, centerLat], 2);
}