The bit of advice missing on giving a target excess fuel level is to first consider knock limitation of the engine.
This is #1 in order BEFORE going for a arbitrary 'ideal' setting on fuel. There is no point targeting one thing if the engine is in 'pieces'
IF you are needing WI in the first place this in most cases means you are knock limited and its very common knowledge that the tendency towards knock drops with 45% range of excess fuel along with WM50 being administered, using less greatly increase the level of knock seen and its pretty poor advice to give to say target a certain point, without mentioning the most important aspect IE: keeping the engine alive.
If you need to see graphs in terms people understand of real applications then I am happy to provide them, I can show you how marginal this is with various levels in Lambda terms from 0.70 to 0.73 to 0.76, it has a large effect.