I thought a bit about how to address the relationship between absolute brightness value (i.e. the value of VCP feature x10) and perceived value in a general way in the libddcutil API. The problem is that the relationship varies from one monitor to another. It may be linear, it may be logarithmic, it may be something else. There are monitors that don't allow values below a certain level (e.g. 20). The cutoff may be absolute, or it is possible that there is another value (e.g. 40) for which, if a value less than the higher value is specified some value greater than the lower bound is set. So the only general way to address the problem is to create a user defined features file for the monitor that specifies the relationship in some way. Alternatively, there could an API call that allows the client program to specify the relationship.
Given the complexity of a proper solution, I decided that this was a problem not worth solving.