So I am worldbuilding a city and came up with some proportions as to the population occupancy. Now I'm a tad-bit stuck at how to convert these to end-point numbers (say I have 400.000 people in there, how many will be Dockworkers?

I understand I can then say: Well 400.000 total pop => 200.000 working pop => 5.000 military pop => 250 knights.

But I am wondering if there is a 'nice' way to do this without explicitly calculating for every single class (i.e. can it be done by linear algebra or another 'simpler' method I'm overlooking? (simpler here meaning less work))