There is your way as one possibility, where initiates gain strength as a proportion of their ultimate potential, or at a certain rate depending on their training. Given your assumption that standard tower training produces identical rates of improvement, I would say it is far more likely that such a cause and effect scenario would produce an incrementally identical rate of gain, rather than a proportionally identical rate of gain.
In other words, in your hypothesis, the Tower's training causes most initiates to gain strength at 3% per year, whereas in the alternative a woman might gain a certain quantity of strength per year. I don't actually believe that Tower training is responsible for the rate of gain, and that is individual, depending on an initiate's capacity, natural aptitude and other factors peculiar to her circumstances. I think the result might LOOK like your proportional hypothesis, but I don't believe it would be that smooth. IMO, stronger women naturally might start out with a greater capacity to channel, and Verin seems to imply as much to Egwene in their first discussion in the tent in Shienar. Nynaeve, for that matter, performed a complex weave requiring not inconsiderable strength for her first feat of channeling. The idea that Egwene's advancement from lighting a stone, to barely igniting a fire to being able to juggle balls of light, to making substantial fireballs in a few months as a result of a steady rise in the percentage of her strength just seems a little off.

Another possibility that seems very probable is that strength is like height - once the process starts, they just grow, baring extraordinary circumstances that might alter their rate of growth (diet, etc for physical growth; forcing for channeling). Once she begins learning or consciously channels, I think a channeler could go off and never train again for years and still reach her full strength at the same rate she would in the Tower. There is no indication that the Kin are in any way lacking in strength despite their strictly enforced policy against pursuing the development and expansion of their abilities.