(Using Blender 3.6.8)
(NB: Answer under work... To be continued)
Approach
Use two cubes as guide, combined with Snap to Vertex activated.
Setup
Using the middle top icons:
- Set Transform Pivot Point to Individual Origins.
- Set Snap to to Vertex, and set Affect to Move and Scale.
Step 1
1. Set the cursor location to a reference position (e.g. the centre of Suzanne right eye).
2. Displace the object to scale origin to the cursor location (menu Object/Set Origin/Origin to 3D Cursor).
3. Add a cube (menu Add/Mesh/Cube).
4. Scale it such that one of its dimensions (e.g. X) matches the specific sought value (menu Object/Transform/Scale).
5. .
Step 2
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Step 3
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Resources: 