I have to agree with several earlier posters that skirting should only be mitred on external corners and scribed on internal corners. An easy way to scribe internal corners on the likes of torus and ovolo skirting is to cut a mitre the "wrong" way as if making an internal mitre, the edge produced where the mitre and front surface meet is your cutting line. Start the cut along the straight bit with a panel saw finishing the curved portions with a coping saw.

At the last house I had to replace every inch of skirting with 7" deep torus and managed every external mitre ( of which there were plenty with chimney breasts and support piers) with a homemade mitre box specific for the skirting. It consisted of two pieces of ply fastened to a spacer a whisker wider than the thickness of the skirting. The left and right mitres were marked on the top edges an carefully marked down the depth of the box and simply cut with the saw I was going to use to cut the skirting. Anyone who does not feel confident in cutting a straight line over a depth in excess of 7" can use a couple of guides affixed to either side of the box to keep the saw from wandering. A couple of pieces of thin ply affixed to the top edges keeps the sides of the box parallel.

Richard