Söderarm is 73nm from wreck, Tallinn 94 nm. The full distance Tallinn to Söderarm is 163nm. The mathematical halfway point is 83nm.* In other words 21nm or 10nm from the Söderarm side of the midpoint and 11 from the Tallinn end.
10 nm is roughly within 6% of the entire journey. As an analogy, if it wass based on one hour, say, rather than distance, then 6% would be equivalent to a time frame of 3.6 minutes either side of the allotted expected time. Which is pretty tight. Bearing in mind the foibles of the waves and the wind speed.
In case you are asking, how come it doesn't add up to 73 + 94 = 167, this is because in
Steve Morse's distance calculator, he adjusts for the fact that latitude is only exactly 60nm per 1º at the Equator. Due to earth being globe-shaped, his distance calculations include an adjustment for latitudes and longitudes further north and south of the equator. So one short journey might add up to X nm and a continuation to Y nm, but the entire journey combined, X + Y might throw out a curve ball result based on this inherent mathematical adjustment.