The z-score table supports multiple charts. You could add a separate chart, called target-weight for example, and fill it with appropriate weight values, for gender and height. The documentation you linked to should provide information about how to edit the z-score document in the database.
Then your form would calculate something like z-score('target-weight', ../my_sex, ../my_age, ../my_height) to display the target weight.
Please let us know if this works for you.