Cleanup: use explicit formula for calculation of translated wordcount

pootle/apps/pootle_statistics/models.py

@@ -638,12 +638,18 @@ def get_paid_wordcounts(self):
         """
         ns = self.wordcount
         s = self.get_similarity()
-        translated_words = ns * (1 - s)
-        if self.rate != 0:
-            translated_words += self.review_rate * ns * s / self.rate
-        else:
-            translated_words += REVIEW_COEF * ns * s / (EDIT_COEF + REVIEW_COEF)
 
+        rate = EDIT_COEF + REVIEW_COEF
+        review_rate = REVIEW_COEF
+        if self.rate != 0:
+            rate = self.rate
+            review_rate = self.review_rate
+        raw_rate = rate - review_rate
+
+        # if similarity is zero then translated_words would be
+        # ns * (1 - s), that equals sum of raw_translation and
+        # review costs divided by translation_rate
+        translated_words = (ns * (1 - s) * raw_rate + ns * review_rate) / rate
         translated_words = round(translated_words, 4)
         reviewed_words = ns