The purpose of a cereal mash is to break down, gelatinize, the starches. In this process you use heat and water, and sometimes some enzymes, to deteriorate the starches to the point where the enzymes in a base malt can turn them into fermentables. In this regard the ratio is irrelevant so long as the base malt in the regular mash has enough enzymes to finish converting itself and the cereal mash.
In wheat the cereal mash is unneeded since the starches will gelatinize at normal sach mash temperatures. The ratio of raw wheat to malted grain is important to ensure there enough enzymes to convert the wheat, which has no enzymes of its own.
Hope that helps.