I think at a commercial mushroom farm they use only 1% spawn - but they also have bulk commercial pasturising rooms where they pasturise the substrate for 3 days, then use large machines to mix the spawn evenly through the substrate before incubating. I think that in our applications, contamination problems are more common and seem to occur at the lower spawn rates. I think you could easily get away with 1:4 but any less and you increase you chances of contamination. Its easy to make spawn so IMO, its better to use a higher ratio rather than losing your project to contamination and having to start over from scratch.