Stallie, spot on, its also about gradients and the last time I calculated it for the average Part 23 type, and it was a while ago, for EN R26 that radio mast to the west was an issue, if it is still there and for R17 it was under or over the Westgate or around which building in the city.
There seems to be a great deal of confusion around here about performance and Part 23 v Part 25.