From an engineering point of view, a straight edge across the front and rear sprockets should be the best result, since it will mean less wear on the chain and sprockets. However, in the real world, in my experience, taht method also means the alignment marks on the swingarm may be out, and the front and rear tyres out of alignment. Its a bit like a "whats the best tyres" thread. Another to try is measuring the distance from the centre of the swingarm pivot to the centre of the rear axle. Ya teks ya pick.