During the historical reconstruction of the Monte Socorro signal mast the "General's view" telescope experiment was matched by a similar visibility test, looking at the window in Quinta da Povóa from which General Spencer would have made his observations, from the foot of the signal mast.

While a modern digital camera has considerably greater power and far more handling advantages than the simpler optics used by the signallers of 1810, you can clearly see that the General (about a mile distant) would have been easily visible through a simple telescope when standing at the window, even in his scarlet jacket, as the enlargement shows - a scarlet flag was draped over the window-sill on this occasion. (Presumably he would have been more easily seen in his white shirt...)

Our thanks to  Colonel Ribeiro de Faria,  Director of the Lisbon Military Museum , for the pictures.