i use RFB 2.0 along with previous versions before it. i almost always manually work the speed dial, and dont rely on the number keys to set pace.
RFB 2.0 has tabs for maximum range at current speed, and total distance for course plotted.
then you just draw a line w/ the nav. tools, and compare this distance with the estimated range at current speed to help determine whether or not you can make it.
the big thing is to determine what speed is the most fuel efficient for the particular sub you are operating,for use this with the above method.
keep in mind that crappy weather and high winds make for choppy sailing, and therefore you need to be a bit more vigilant maintaining that speed.
good luck.
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