Fictitious Secondary Station problem

"To get the height differences you should also interpolate for high tide between 0.2 meters (Spring) and -0.2 meters (Neap) for the high tide and since we are closer to Neap intuition would say about -0.1 meters. And for low tide between 0.2 (Spring) and 0.5 (Neap) - intuition = about 0.4 because we are closer to Neap. Thus on your tide curve the low tide number would be Frazer minus 0.1 meters and for high tide Frazer plus 0.4 meters." This reasoning is puzzling: For the interpolation which low tides at the Port are being used: 1.6 or 1.0; and similarly 3.3 or 3.6 for the high tides. The problem is, you can't use interpolation for the 1.6 or the 3.3 because they are outside of the ranges specified for the secondary station (for low: 0.4 to 1.1; for high tides 3.3-3.6). Which tides of the fictitious port are being used? Or are we deriving the height based on if the time is in the middle between the neap and spring high tides then it should directly in the middle of the neap and spring high tide heights?