Real geologic data are rarely so neat. You must be able to interpolate when the data are not in round numbers. Say your outcrop is at an elevation of 871 meters. The 800 meter contour lies this far from the outcrop - in the downdip direction because it's lower than the outcrop. The 900 meter contour lies .29S in the updip direction, because it's higher.
Linear interpolation is always valid between any two points in a plane.
Mathematically valid, that is. Interpolation will not detect some geological interruption of the structure, like a deep channel fill or a fault. Construct the structure contours on this sill. Determine the contour spacing, either by trigonometry or by drawing a cross-section. Note that graphical construction is not as precise as trigonometrical calculation.