Embroidery Library have really useful resources for advice on hooping, stabilisers and placement. The placement guide is at
http://www.emblibrary.com/EL/elprojects/Projects.aspx?cs_productid=pr1152&cs_catalog=elprojectsI usually start from their guidelines and then adjust by eye. If it is a repeat order, I keep a note of the measurements so they are exactly the same. I also find quilter's acrylic rulers useful, I use 2 to set the centre horizontal and vertical lines and slip the paper template underneath, then hold it down with masking tape and hoop it up. Once it is on the machine, check the needle is going to hit the centre dead on and adjust if necessary. If the fabric is slippery or would be marked by tape, I mark the lines with tailor's chalk or a vanishing pen instead.