It is sometime useful to look at the percent resonance (%r), because r is dependent on the reaction and is the same for all substituents.
One can predict the difference in data comparing two substituents using %r:
The most dominant effect is clear when looking at the ratio of R to F.  For example, a tungsten complex was shown to alkylate allyl carbonates A and B.  The ratio  of products A1 and B1 can be attributed to the para substituent, X (Fig. 3).  Using Swain–Lupton parameters (σ = 0.2F + 0.8R) a ρ value of -2.5 was found to be the slope.
This is in agreement with the proposed mechanism (a positive charge forms on the benzylic carbon and is stabilized by resonance; R dominates by a ratio of 0.8/0.2).