An f-stop in this context is a quantity of light. It can be achieved by opening the lens further or it can be achieved by adding time. I prefer adding time when dealing with long exposures. So if the indicated time is more than one second but less than one minute, double the time. If the time is more than one minute, double it and double it again - i.e. if the meter says 1 minute, then give it four. if it says 15 seconds, give it 30 seconds. This is a fairly stable formula for most color negative emulsions. This is probably why Kodak has not provided a chart for Ektar. Besides, once you get into these extremely long exposures, precision becomes a relative thing and is fairly irrelevant. What you get is largely a matter of taste and need, not a matter of accuracy.