Rear Speakers listening position

[Dr.Rock]

My surround package includes M7DS rear speakers with a 6.1 amp. At the moment it's setup as a 5.1 with the surround speakers on the side walls 3m apart from each other and 1 metre away from the back wall. I am looking to put a pair or surround-back speakers on the back wall and wondered if there's a rule of thumb for how far they should be apart from each other and from the side walls?

ie, should there be an equal distance between the surround-left and 1st surround-rear, 1st surround-rear and 2nd surround-rear, 2nd surround-rear and surround-right, or what?

I need a rough idea before I start drilling holes into my wall.

Thanks.