Am I allowed to take measurements before declaring an attack?
The rules don't specifically state this. 

In the example on page 4 of the rule book it notes that you declare targets before rolling dice and specifically points to an example where multiple ships are in an arc and this rule is used to prevent players from determining new targets based on the effects of shots. Since it says nothing about range and arc calculations nor about choosing targets and then not be able to retarget if they are actually our of arc or range it would imply validity for arc and range are determined before declaring the attack.