You can't make a Mack snow without the Mack snow gene. Therefore one or both of your leos must be carrying it to start with. Even if only one parent leo is carrying the gene you will still hatch out Macks, although I'm not sure what the percentage of that would be!
All a leo needs is one Mack snow gene to be a Mack snow. So you have to start with a leo that has a Mack snow gene. Crossing it to anything else produces some babies that are Mack snow, but not all the babies would be Mack snow.