I’m no professional player at all, but it seems like a balanced rack could be keeping a certain number of vowels/consonants. Considering there are 100 letter tiles, 56 consonants and 42 vowels, with a ratio of 56:42 or 1.33 consonant:vowel ratio, keeping a ratio as close as possible could be good maybe. Here is my idea for ideal consonant:vowel ratios making it as close to 1.33 as I can:

6 tiles left: 3 consonants, 3 vowels; 1 ratio

5 tiles left: 3 consonants, 2 vowels; 1.5 ratio

4 tiles left: 2 consonants, 2 vowels; 1 ratio

3 tiles left: 2 consonants, 1 vowel; 2 ratio

2 tiles left: 1 consonant, 1 vowel; 1 ratio

1 tiles left: 1 consonant; indefinite ratio

I know it heavily depends on the current board as well the tile bag, but for general purposes, what would be a good ratio for each number of tiles remaining at the end of the turn?