the formula for determining object scaling is:
Object Height x Annotation Scale x Annotation Plot Size = Model Space or Paper Space Height In this case, we want the room label text height to be 3 mm in paper space when plotting the drawing.
Assuming an annotation plot size of 1 mm and an object height of 300 mm, we can solve for the annotation scale as follows:
300 x Annotation Scale x 1 = 3 Annotation Scale = 3 / 300 Annotation Scale = 0.01 Therefore, the viewport scale that corresponds to this annotation scale is B. 100:1.
https://knowledge.autodesk.com/support/autocad-architecture/learn-explore/caas/CloudHelp/cloudhelp/2016/EN