How to set units when creating paths with a script?

Question

I'm trying to update a script that used to work in Photoshop CS4 but which seems to completely ignore the units I'm using in Cloud.  The script below should create a path layer with two lines that form an X across the entire document. 
To test, I'm creating a new page that's 8.5 x 11 inches and 300 DPI.  I then run the script.  However, the the shape that is produced is huge and most of it is offscreen.  What am I doing wrong?
function Point(x, y)
          this.x = x;
          this.y = y;
function addVertexCorner(lineArray, x, y)
          var p0Info = new PathPointInfo();
          lineArray.push(p0Info);
          p0Info.kind = PointKind.CORNERPOINT;
          p0Info.anchor = new Array(x, y);
          p0Info.leftDirection = p0Info.anchor;
          p0Info.rightDirection = p0Info.anchor;
function appendLine(p0, p1, lineSubPathArray)
          var lineArray = new Array();
          addVertexCorner(lineArray, p0.x, p0.y);
          addVertexCorner(lineArray, p1.x, p1.y);
          var pathInfo = new SubPathInfo();
          lineSubPathArray.push(pathInfo);
          pathInfo.operation = ShapeOperation.SHAPEADD;
          pathInfo.closed = false;
          pathInfo.entireSubPath = lineArray;
function createPathLayer(title, subPathArray)
          var docRef = app.activeDocument;
          var originalUnit = app.preferences.rulerUnits;
          app.preferences.rulerUnits = Units.PIXELS;
          var myPathItem = docRef.pathItems.add(title, subPathArray);
          app.preferences.rulerUnits = originalUnit;
var docRef = app.activeDocument;
var imgWidth = docRef.width.as("px");
var imgHeight = docRef.height.as("px");
var p00 = new Point(0, 0);
var p10 = new Point(imgWidth, 0);
var p01 = new Point(0, imgHeight);
var p11 = new Point(imgWidth, imgHeight);
var lineSubPathArray = new Array();
appendLine(p00, p11, lineSubPathArray);
appendLine(p01, p10, lineSubPathArray);
createPathLayer("Big X", lineSubPathArray);

Answer 1

I did, andd while that did help to center the image on the page, now all my measurements are coming out about 4 times too small.  I'm also worried that changing the resolution might cause the image to lose some data due to resizing.
What exactly does changing the resolution do?  Why does this center things?  How can I calculate points so that they exactly touch the edge of the document?