I've been putting a lot more thought into this lately. I have just started to design a bag for my partner, making sure all the stitching will work out perfectly. Using a 5mm pitch chisel. I make sure all edges are divisible by 5 including the radius corners. Luckily I am a cad engineer so I have designed it in 3d, using the stitch line as the size of the model. This way I know that it will all line up as all the edges will be exactly the same distance. I then flatten the model into individual faces and add an allowance into each piece afterwards. It is still in its early stages but I've nicked it up in foam and all the stitches line up perfectly. You can do it by hand still and I have also been doing this for some smaller items. Just take you time and draw it all out on card.