Some boards (like the Dragon 13'9" ) I like heavier and some lighter. If the design is good then weight isn't an issue unless it reduces the buoyancy too much, so it is related to board volume. I don't use a length to weight ratio. I've gone up to 70 pounds on the Dragon, and it handles everything I took it out in including triple overhead ledging reef waves, but it isn't for the faint of heart. The Dragon I built last year could have been made down to about 35 pounds but I beefed it up to 50. For boards up to 12 feet there's no harm in going as light as you can though.
Malibu type shapes are very sensitive to weight changes because of the imbalance of the aft turning position, more balanced designs with a more central riding position handle weight better.
If you want to go light a 1/4" deck and bottom with lightweight cloth under the deck ( I use nylon) with 3/8" frames at 4 inch centres and less rail blocking inside the board is the way to go. I've been using paulownia which makes life easy in this respect, but have used heavier woods in the past. Denser woods can be used with lighter scantlings ( thickness etc) but this does require accurate work.