Consider following backward design: first, identify your goals; then, determine how you will evaluate them. What do you hope the students can do that this tool assesses? In my mind, it's impossible to say how you should break down your translation exercises without knowing this.
Some possible options...
- By the end of this course, students will be able to understand the central idea of a text in a source language and render this idea in a target language.
If this is your goal, the basic unit of the passage to be translated is the semantic one. Tally up the ideas you think are the most important. For example, taking this sentence of Dickens':
“There is nothing in the world so irresistibly contagious as laughter and good humor.”
A translation would have to capture (i) laughter is "contagious", spreading easily; (ii) nothing compares to it for contagiousness. You would deduct marks for such mistakes as applying "irresistible" to the laughter itself or saying that there's nothing in the world like a laugh.
- By the end of this course, students will be able to reproduce in a target language the idiom and style of a text in a source language.
If this is your goal, the basic unit of the passage is the sentiment: the imagery, the positive/negative skew, the connotations. For example, taking this sentence of Dickens':
“I loved her against reason, against promise, against peace, against hope, against happiness, against all discouragement that could be.”
A translation would have to capture (i) the repeated rhythm of "against X"; (ii) the startling contrast between the positivity of the first five abstract nouns and the negativity of "discouragement" that is intended to summarize them.
- By the end of this course, students will be able to identify and effect various types of modulation and transposition in translating a text from a source language to a target language.
If this is your goal, the basic unit of the passage is syntactic group: noun phrase, verb phrase, adverbial complement, etc. For example, in this sentence of Camus':
“En entrant, Cottard et Rambert essuyèrent leur front.”
As they entered, Cottard and Rambert wiped their foreheads.
A translation would need to (i) avoid rendering "en entrant" awkwardly as a participle; (ii) make sure the singular "front" ends up as the plural "foreheads".
- By the end of this course, students will be able to do all of the above...
Then your rubric (or discrete rubrics for discrete exercises) can capture all those aspects under various headings, or whichever other ones correspond to your goals.