If the book doesn't give you much to go on, a useful way to determine an approximate zero thrust setting for the conditions is
(a) feather the engine and trim out for the desired speed
(b) restart and adjust the "failed" engine until the same flight handling and performance is achieved at the same density height.
(c) do this for a few test points and you have your zero thrust schedule nailed.
Needless to say, this exercise needs to be done on a nice day with no turbulence ...