The most important parameter to consider when you decide upon your
chopper frame is the trail.
We at Unlimited Choppers recommend a trail between 2-5 inches.
Staying within that limit will ensure a good handling chopper.

Here is an online calculator thanks to Perse Performance

To check your trail please do the following:
a) with a measuring tape or a straight stick draw a parallel through the
steering head and down to the ground
b) Then draw another line from the center of the front axle and straight
down to the ground.
You should now measure the distance between the two points,
which is the actual trail of your bike.
The actual trail and the distance should be between 2-7 inches

To calculate your rake, front fork and trail there are a few values that need to be known.
For future reference I will refer to them as follows:

 

L = Total length of front fork measured from the bottom of the upper triple tree to the center of the front axle
R = Radius of tire and rim
a = Angle measured from a straight line down from the steering head to the center of the fork tube
b = Rake in triple tree
D = Distance from a straight line through the center top of the front forks to the steering head axis.
measured from the top of the triple tree
T = Trail of the bike
H = Distance from the center of the upper frame neck straight down to the ground.