You can estimate average accuracy for modern wheels, but accuracy varies considerably between wheels.
Nobody ever bets 28 numbers. Typically you bet 5-12 or so numbers. It depends on the wheel and settings you use.
There are many different ways to apply visual ballistics, and none of them are better than a computer unless you are a master at vb. With a computer, you can virtually instantly do what may take someone weeks to learn, and even then the computer is better.