Gustave has taken it upon himself to observe Marion and Sanglain following the death of the Princess and DT.   But KT thinks Gustave is in the city of Marquesas for other reasons. 
By your own count means 
it's all made up.
Pero por tu propio conteo, 
quiere decir que es inventado.
Esa cifra tiene pinta de que te la has sacado de la manga.
Pela sua própria conta,
isso é tudo inventado.