Start with your lighter color first and spray past where you want the fade to end, then creep up on your fade outline with your darker color.
As far as how well defined you want your faded edge to be, that all has to be determined how far off the surface you're placing your "soft" mask, or cardboard.
Tape spacer blocks to your boat to hold up the posterboard evenly to the surface. Poster tack would work good too, instead of tape. Sort of mold the posterboard around your hull like a heat sheild, and use poster tack to hold it down to your spacer blocks. The spacer blocks can be anything as long as they are equal, like a set of dice or wood cutoffs. Keep the spacer blocks as far back as you can under your masking edge to prevent them from affecting the pattern, and to also keep their outline from showing up.
A good tip with using rattle cans to get a finer mist, is fill a pan of hot water and let the can warm up to it before you use it, and always shake it good and keep the tip clean. Never "spot" spray with it or you'll pick up small splatter spots that won't match the finer mist.
Always start farther back in the air with your can, and gradually work up closer with it to build up your color. You have to keep the direction of your spray blowing past the edge of the mask, never try and blow under it. The more shallow of an angle, the softer the fade. The more direct you spray downward on the mask, the sharper the fade, and the more risk of it overspraying under your mask, and outlining your spacer blocks or such.
Experiment on a piece of white posterboard first before you get brave with your new boat, then after you get your technique down pat, use the posterboard you practiced on to cut your soft masks with
