使用画布垂直和水平居中文本

I'm trying to align a text in bitmap horizontally and vertically, I read several post but I can't find a solution. The bitmap is a simple image of a circle. I post my current code. More or less it works but the text is not perfectly centered, it seems a bit on the left and a bit on the top, I mean it seems I have to add an offset to move it to the right and to the bottom.I'm trying to align a text in bitmap horizontal