This is pretty open. I use a 50-50 mix, but I don't measure carefully. I think you could drop down to 30% glue and still have it work for small ground foam.
The important thing when doing this is spray down a good "professional" wetting solution Again this is pretty loosely defined, but is a few drops of liquid detergent or soap in maybe 12 ounces of water. This breaks down the surface tension in the glue mix and allows it to spread.