Iron starts to rust at 50% RH and steel is around 80% RH. If we are talking about protecting metal the lower the humidity the better but as @rbscebu says we must also consider the scales. 50% is recommended for preservation of historic artifacts made of ivory, bone, horn and shell. This is the upper end for iron but safety below the threshold for steel. For wood it is generally recommended to storage near the average ambient humidity of your location. In Melbourne, Australia the yearly average is 50% but there are big daily and seasonal swings.
For plastic scales I think it makes sense to go as dry as possible <50%. For natural scales 45-50% is a good number to shoot for. If natural scales get too dry they will tend to crack and become brittle. Constant changes in humidity are also no good. Stable humidity and temperature are best.
It is important to remember that relative humidity is directly related temperature. It is a measure of how saturated the air is prior to reaching the dew point. The total water content of a sealed container may be constant but the relative humidity will change (drastically) with temperature. To keep humidity stable you must also maintain a stable temperate or have a means of extracting or inserting moisture from/into the air as the temperature changes. This is where desiccants like silca gel or saturated salt solutions are useful. They have the ability to adjust the water levels in the air to maintain stable conditions. The larger the surface area, the faster the adjustment. The greater the mass, the the larger volume of air it is able to control.
