How to override ARRegister.SignBalance calculated property

I am working with the standard DAC ARRegister and need to change the logic of the calculated property SignBalance.

Standard implementation:

public virtual decimal? SignBalance

{

    [PXDependsOnFields(typeof(docType))]

    get

    {

        return ARDocType.SignBalance(_DocType);

    }

    set { }

}

My requirement is to have SignBalance calculated using custom logic, i.e. the getter should call my own method instead of ARDocType.SignBalance.