I've made this little spreadsheet to allow me to quickly make a population estimate; just input the number of province of each level (split between low, medium and high part of the population range) and you get total population out.

For example (included in the sheet): Bellam has 1 low pop province (4), 1 medium pop province (4) and 1 low pop province (5). Gives a total pop of about 160.000