Why stickout has such an outsized effect
A cutting tool sticking out of its holder behaves like a cantilevered beam, and deflection on a cantilever does not scale gently with length. Small increases in stickout produce disproportionately large increases in deflection, which is why a tool that cuts perfectly clean at a short stickout can suddenly chatter, deflect, or even break once extended just a little further to reach a deeper feature.
How the numbers are calculated
Length-to-diameter ratio is simply the stickout length divided by the shank diameter, a quick way to judge how aggressive a given setup is. The relative deflection factor uses the cantilever beam approximation, where deflection scales with stickout cubed and inversely with diameter to the fourth power, normalized here to a value of 1 at an L to D ratio of 4, a commonly cited comfortable baseline for rigid cutting.
Practical tips
Keep stickout as short as the feature actually requires rather than defaulting to a long-reach tool out of convenience, since even doubling the stickout on the same tool can multiply deflection by a factor of eight due to the cubic relationship. When a long stickout is unavoidable, back off feed rate and radial depth of cut to compensate for the reduced rigidity, and consider a larger-diameter tool or a reduced-shank design that keeps more of the shank at full diameter closer to the cutting edge.