Hi Pongo de Mer!
Welcome to the Campaign Series!!
Look at the box directly to the right of that turns box. It will say visibility.
Whatever you pick for the "Turns" then the visibility has to say the same for turns!
Example: Say the scenario was 1-30[d] turns and you changed it to 1-16[d] turns. Then over at the visibility box it has to say the same. If it(the visibility) box still says 1-30[x] then you have to change it to 1-16[x] also.
For further information the [x] in the visibility box is the actual visibility in hexes in any scenario. You can change that to any number from 1 to 20. That signifies the actual visibility in any scenario can be from 1 to 20 hexes depending on your choice.
In the Turns box the 1-16[d], the [d] signifies daytime. If you change that to an [n] then it signifies nighttime.
I hope this helps my friend! If your still having problems post here on this forum.