The key here is the phrase:
Codex Necrons, p15 wrote:
...are then immediately placed back together anywhere on the table top using Deep Strike rules.
Emphasis mine
The unit isnt placed back into reserve (like eldar swooping hawks are) and so no further reserve rolls are made. The exception to this is if they are effected by a Deep Strike mishap and roll 5 or 6 on the table. In this case they will be placed back into reserve and then you will have to roll as normal. You have been playing it correctly.
In answer to your other question, the only times that you are allowed to measure during a game is when it specifically allows you to. Checking movement distance while moving and assaulting, checking distance after delaring shooting and so on. You cant arbitrarily measure stuff as that would give you an unfair advantage.