The most important number to indicate Carr's progression will be his third down conversion percentage. The main problem on offense that the Texans have had their first two seasons is not being able to sustain drives. Because of the conservative nature of Capers (even though I am sure Palmer would like to open the offense a lot more) other stats like TD's, yardage, QB rating, and even TD-INT ratio will not be as important to Carr.
If the running game is going really well, Carr will not have as many TD's as the top QB's because Capers will want to run it in. That fact alone will effect Carr's TD-INT ratio and QB rating. A great running game will keep Carr from racking up big yardage numbers, although an improved running game will help open up the passing game when passes are called.
Also, Carr is going to naturally throw more INT's than QB's in West Coast offenses because the Texans offense depends more on downfield passes than on the safer short-yardage passes.
Texans fans need to realize the limitations the offensive scheme is going to put on Carr when they look at his stats. Of course he needs to limit his INT's and complete more of his passes. But more important for his progression will be how many third downs he is converting, how much time he is running off the clock, how many points the offense as a whole is scoring, and ulitimately how many wins the Texans are getting.