The basic rack is really a Virtual shape that can be used with modifications to define both the tool and the gear. There is enough information about the basic rack that you can define both.
It sounds like what you are asking about is the basic rack fillet radius. On the basic rack, the fillet radius is a simple circular arc of some size r. The value of r will correspond to the size of the radius on the tip of the cutter. On the actual gear, the fillet is a trochoid shape which is a special shape created by the sweeping path of the cutter tip that has been radiused. The trochoid has a varying radius throught its non circular arc.
When we specify fillet radii on gear teeth, what we are really specifying is the fillet radius on the "Virtual" basic rack.
The most commonly used North American standards for 20 degree pressure angles are.
For fine pitch gears - a 0 basic rack fillet radius - although it is customary to give it some unspecified value - I know it really sucks doesnt it
For course pitch gears .38 x the module
And generally available for all gears
a full fillet radius - this gives the lowest stress conditon
I would suggest using a full fillet radius whereever possible for the lowest bending stress. The only disadvantage is that this tends to make the form diameter larger - which usually isnt a problem, but on designs which are marginal in contact ratio, this could be a challenge.