Isn't this really just a pretty simple calculation? What's with the W&P slide rule thingy? If you want to know if you have enough fuel to get out of the bay, and you have 60 gallons and your burn rate is .75 at 6 knots, you can run for 80 hours. 80 * .8 = 64 hours with a 20% reserve. 64 hours * 6 knots = 384 nautical miles of range at 6 knots. If it's 100 nautical miles to get out of the bay, you have plenty of room. It seems to me that that you tried to make it more complicated than it is.
Isn't this really just a pretty simple calculation? What's with the W&P slide rule thingy? If you want to know if you have enough fuel to get out of the bay, and you have 60 gallons and your burn rate is .75 at 6 knots, you can run for 80 hours. 80 * .8 = 64 hours with a 20% reserve. 64 hours * 6 knots = 384 nautical miles of range at 6 knots. If it's 100 nautical miles to get out of the bay, you have plenty of room. It seems to me that that you tried to make it more complicated than it is.
So no need to account for currents and crosswinds.