Separately, someone has advised me on the depth of field considerations, based upon circles of confusion. If the depth of field allowed for the given circle of confusion limit at the negative side of an enlarging lens is the same as in a camera, its going to be of the order of 0.3mm. When enlarging by a factor of 5x, this leads to a depth of field allowed on the base board of 1.5cm. By my maths, that means 7degrees for the short side of a 5x7, or 5degrees on the long side.
I looked into some advice yesterday regarding split grade printing on Les McLeans website, and it had some great tips, making it seem trivially simple. Separately, I have a EM 10 winging its way in the post.