// Get Page width & height in inches
double pgCenterX = (page.width / page.resx) / 2;
double pgCenterY = (page.height / page.resy) / 2;

// Get Layer half width and half height in inches
int nOld = Layer.unit;
Layer.unit = 2; // Convert to inches
double layHalfX = Layer.width / 2;
double layHalfY = Layer.height / 2;

// Center Layer horizontally (X)
Layer.left = pgCenterX - layHalfX;

// Center Layer vertically (Y)
Layer.top = pgCenterY - layHalfY;

 // Restore previous Layer unit value
Layer.unit = nOld;