Tanka consists of two sections. One is the objective observation of something affecting the poet and often expressed almost like the haiku that derived from it. The second is the emotional and personal reaction of the poet to the described matter, a very subjective statement. The blending of these two into a cohesive whole make the tanka.
Forget about syllable counts and other frivolities. Japanese kanji and other linguistic elements do not compare closely to syllables. It is better, as in haiku, to use the smallest number of simple words.
For form's sake we continue to use a five line layout. If the first segment is the objective, the haiku observation, the latter should be the subjective and emotional expression. The middle line may often become a turning point between the two, a part of both observations, but again there is no steadfast rule.