I don’t think that people understand the meaning of proportionate. Half the population is female, so that should be reflected at every level of senior jobs. However, half the population is not BME, trans or disabled but I have seen people arguing for 50% representation of these groups before. I think that any organisation with a commitment to diversity should aim for its workforce to reflect the general population.
I think there can be a few challenges to being very strict with these proportions though. Even apart from questions around whether some industries have reasons other than sexism for an imbalance compared to the population.
Even in an industry that as a whole has proportionate representation, it won't follow that each particular workplace will have the chance to hire exactly proportionately. Over time you would tend to see a certain amount of fluctuation in either direction. If you don't build that flexibility into a quota system it will tend to lead to problems where you may be stuck not filling a position, or hiring someone significantly less competent.
It's also simply not within the control of an individual business to create all the conditions that would lead to them having the right people applying. An engineering firm that said they would hire 50% women cannot do much about the fact that 50% of the engineers graduating are not women. Whatever the cause of that is, it exists at a different place in the system and has to be addressed there.
I think there is also some question, within industries, of what sort of positions appeal to various people. Why do women make up such a large proportion of GPs, but not surgeons? Does it matter?
Making a target like 40%, in an industry like law with a lot of female graduates, is pretty reasonable I think. It allows for enough flexibility to accommodate particular circumstances, but its a pretty robust target that will make a long term difference in how they think about hiring and management.