Say we need 33% optimal bluff frequency in a spot.  We bet 10 combos of value that have 80% vs an opponent's calling range.

Do these 10 combos turn into 8 value combos and 2 "bluffs"?

If it does, to get to 33%, we need to add 2 extra combos of bluffs.  Totaling 8 value combos, 2 "bluffs" (losing value hands) and 2 extra combos of bluffs.  8 value, 4 bluffs, 33% optimal bluff frequency.

However, vs our opponent's pure bluffcatchers that all our value hands beat, we still have 10 value vs 2 bluffs, so only a 17% bluffing frequency.  This seems wrong.  So I am wondering what exactly do we need to consider on river bets.