with a Sat. Nav. I would do my running DR and plot it when the Sat. Nav. got it's hit. It took about 15 minutes for it to calculate the doppler effect and give the position. In that 15 minutes I would make the plotting sheet, do my calculations (or advance the previous plot)and plot my DR on the sheet. When I got the position off the Sat Nav. I would plot it and compare it to my DR. I would do the same with the sextant fix(s). It was a lot of fun and gave me something to do. Your right, over time you can chip away at the error and get pretty close. It's funny how things change when you get a "little older" and gently slide into the lazy mode. I may have to drag that stuff out again and play around a bit just for grins.