According to the calculator you'd be able to get up to about 57.7 mph with 20 inch tires. That is assuming you weigh absolutely nothing and you're ridin in a vacuum.
High front, Low rear = Speed
Low front, High rear = Acceleration
But remember, you can't have both. Find what you want, it will depend on your style of riding. If you ride MX or trails a lot you'll probably want more acceleration. If you have a lot of flat straights you might prefer a gearing that'll get you more speed. There is no one best gearing.
With taller, high speed (15-36) gearing you have to have power to "pull" it. A stock blaster might not be able to get out of 4th gear and accelerate like a snail. Low gearing (12-44) will accelerate like John Force, and top out at about 55mph. Can't really have best of both worlds. X(
You have to decide do I want more speed or do I want to get to speed quicker? Best way for more speed is to start modifying your engine. I:I