There is nothing wrong with using the word preferably in relation to numerical values. You may have used in a context that raises clarity issues.

There is nothing wrong with using the word “preferably” in relation to numerical values in the specification.

However a claim needs to define a boundary between what it covers and what it does not cover so there is no place for that word in any claim. Does a product infringe that doesn’t meet the “preferred” values?

You can use “about” and “an effective amount”.

You can have an independent claim with the broader values and a depended claim with narrower values. Each clear but one a subset of the other.