It perhaps depends on what is being counted as billable.
For example, if I'm briefing a junior, checking their work, and keeping tabs on their progress that's not me doing the actual "useful" work, but is work which can be charged for as without it the work can't be undertaken.
From what was being described the previous poster was describing those sorts of tasks.
In which case, that appears to be a sensible target. In that you don't want a highly paid individual only supervising staff, you need them to be using their skills and experience on "useful" project work.
If that's the target:
30% general admin work, training, leave, etc.
35% project management work (still invoiced but just not "useful" in terms of using technical skills)
35% true billable (i.e. bringing their full technical potential to the project)
That sounds a very useful target to be working towards, as it limits the risk of senior staff spending all their time doing project management work and not using their technical skills.
That's not to say that the project management isn't important, but some of that could be delicated to others.
For example, a graduate engineer presenting a set of levels could have nearly all of them checked by a senior engineer (with "working non printing information" embedded in the design packs) and so the principal engineer just needs to use their experience to look for numbers which "look off" (obviously logging those they check, again using working non printing information) and the director would be able ask about the levels checked by the senior staff and so may only pick up a few (although ideally none) which they have concerns with for them to be checked before being issued.
By using the non printing information, the graduate knows where they've checked between, the senior engineer can be sure that the numbers used are right, the principal engineer can log which they've checked and the director can be confident that it's all to a good standard.
If at every check the maths was done again by the next person checking the work that would take up a lot of time and whist could be "billable" isn't useful billable* as it would be better for the principal to be working on another more complex project and the director working on winning new projects (the key time to do so is when you're at your busiest, as if you don't you'll have slack time soon after).
*it'll also mean that others will be more lean and so can keep their fixed cost fees down and so will win more work.
All in all, if seams a highly sensible target to work to.