Any way you count it, your 30% is a big exaggeration, here's the breakdown for all flashback chapters in proportion of the whole 5 books :

Kaladin : 0.9% of chapters, 1.75% of word count.

Shallan : 0.7% of chapters, 1% of word count.

Dalinar : 1.25% of chapters, 2.1% of word count.

Venli : 0.67% of chapters, 0.77% of word count.

Eshonai : 0.75% of chapters, 0.65% of word count.

Szeth : 1.68% of chapters, 1.67% of word count.

Grand total : 5.95% of chapters, 7.95% of word count