A vector diagram for course to steer in a cross-tide is only a diagrammatic representation that depends on proportion. You don’t need necessarily to use your dividers to measure miles, especially when the scale is inconvenient and refuses to fit the passage leg or the chart. Try using the subdivisions on the side of the plotter instead. It doesn’t matter which ones you choose, so long as tide and boat speed get the same treatment. Five divisions for five knots, etc. Select the scale that suits your needs on the day. It’s quick and it’s easy.