Cocktail Rye

I’m considering doing the Cocktail Rye recipe found here: http://www.thefreshloaf.com/node/38551/100-rye

I’ve purchased the 3 loaf lasagna pan, as well as a can of Tate & Lyle’s Black Treacle as an added sweetener.

I’m wondering as to the amount of the black treacle to add to the recipe? Checking some other rye bread recipes that add a sweetener (honey) and it seems to be in the vicinity of 8%.

Any thoughts on how much to add? Looking for something with a hint of sweet, but not overly sweet. Thank you