Tips for Building a Thin Master from an Existing Black Weight

To confirm: you have only a Black weight?

It is quite challenging to go all the way from Black to Thin without at least one intermediary master, especially if you want all the stages between them to be viable. This is one reason a lot of designers start with the Regular weight, and then derive heavier and lighter designs from that (the other reason is that the Regular weight tends to be the most widely used and embodies the character of the typeface for the user).

One approach to the situation you describe is to work on a Regular weight intermediary master next, and then use that to extrapolate a rough Thin master. The latter will still require work to refine the design, and get its proportions how you want in relationship to the Regular (Black designs tend to need the most optical adjustment to stroke and bowl weights and counter widths, while the Thin can be much more systematically related to the Regular proportions and stroke weights).