@R.Portman Welcome to the forum, Roland.
I canāt speak for Sienci on the subject of providing a mounting template, but I can say that there really is no āproper locationā for the rails on the table top. You can mount the rails any way you like on your table top, so long as the Y rails are parallel to one another and they are square to the X gantry. You may have already seen this, but here is the Sienci instruction on mounting the Mill to the table:
As you can see by that article and as youāve likely determined through your own experience, you could mount the Mill diagonally on the table top if you wanted, again so long as things are parallel and square.
In terms of getting things square, here is one thread on this forum on a method to square things up:
If you search this forum for āsquareā, you will find several others.
I hope this replies to your question. If not, please explain how you think Sienci can improve on this and Iāll take it up with them.