A typical CAD approach is to draw linework at actual size and then add annotation. If annotation is added to model space then it is scaled appropriately so that it appears the same size on the scaled sheets regardless of the scale used. Based on your description, I assume that is the approach used at your company. For that approach, you should use the built-in CAD feature of annotation scaling and annotative objects.
I also assume that you want the length value that is displayed to be the actual length of the line and that you want the diamond with user editable tag number and text to scale based on the scale of the drawing. In that case you will need to make the main block itself non-annotative. You will to make the user editable tag annotative within the main block with justification middle/center. You will need to make the diamond into an annotative nested block within the main block with the center of the diamond at the origin in the nested block definition.
For rounding to the nearest 6", you will need to use the formula field code:
round(X/6)*6
Replace the X with a nested field code of object, select the line, select length, check the box on the bottom right that says 'display value for block reference'.
