I would read the paragraph above and below 34a/b more carefully.

I'll paraphrase:
"t shall be no less than 34a or b when you have D or d."
"You may use 35a/b to correlate design pressure with D, T, co, ci, etc."

Let's change the subject matter, but use similar wording:
"If you pick the wrong thickness, the amount of money you owe me shall be no less than $5000."
"You may use this formula to correlate %error and the amount of money owed to me..."