The mist coat ratio is anywhere between 50/50 to 75/25 paint to water but it depends on the viscosity of the paint. As a guide it should resemble single cream. Yes, rub down between coats and check for imperfections and fill these before the final coats. Any filler should also be touched in with the mist coat mix before top coats. The number of coats really depends on coverage, keep applying until you have good coverage and you can't see the cutting in. You should find it covers with 1 mist coat 2 full coats.
Hope that helps