[Deep breath]
When you find the lowest point that the suspension can droop without the axleshafts binding (you'll probably have to clearance the beam 'windows' and trim the center slip spline) measure the distance from top of the spring bucket to the spring mounting surface on the axle. This will be your desired free length or unloaded length of your spring, let's say this is 22". Now, you'll also want to take the same measurement at your desired ride height, we'll call this 15". Take the difference between these two (22" - 16" = 6") This will be the distance that the spring compresses, this is important later. We'll also need to know the weight over the front axle of your truck. Lets say that the total weight is 3500#, and 60% is over the front axle, 2100#. Subtract the weight of the tires, rims, and axle and you get about 1800#. Divide that by two (since you'll have two springs supporting this weight (1800# / 2 = 900#). This is the tricky part that many folks overlook. The distance between the beam pivot and the wheel mounting surface is nearly 1.5X the distance between the beam pivot and spring pad. This gives us our "TTB Nutcracker Effect". So we now have to multiply our weight by this ratio (1.5 x 900# = 1350#). This is now the force that the spring will have to excert in order to maintain the desired ride height. In order to determing the most desireable spring rate, you want to take that effective weight or force (1350#) and divide it by the number of inches of compression that the spring sees at ride height (6"), so (1350# / 6" = 225#/") Using these example numbers, I was able to get a desired spring rate of 225 lb/in.
[/deep breath]
You'll need to determine the actual numbers for your specific truck with its specific modifications, but this will give you an idea of what goes into "tuning" the TTB for more flex. Then, you'll have to measure for custom shocks, and probably want to do some rear suspension work as well, but that's a different class.
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