I agree with the previous posters that it's best to plan for the particular SPI you will be using when you make your pattern. That being said, if I'm just putting something together quickly I usually don't . To improvise on the fly, Twin Oaks's method is correct. Another thing that will help is that I never go all the one from one corner to another if I'm not sure my stitching will fit perfectly. So start on one side, work almost to the middle, then move over to the other side and work towards the middle. That way, you know your corners are going to be perfect and you can figure out where to split the difference in the middle where it doesn't matter as much.