Tighten lightly -- only 2 diagonal screws to hold plinth in place. Measure. If not equal, loosen 2 diagonal screws. Adjust plinth as necessary. Tighten 2 diagonal screws lightly. Measure again. Loosen and adjust plinth again, if necessary. Tighten.
Keep repeating till right. When good, securely fasten ALL screws.
The whole thing -- both speakers -- should take less than 5 mins. Seriously.
(I'm assuming it's done with speaker laying on it's side for entire process, propped up on milk crates or similar.)