I don't know if this has been posted before, but ObsidianMD with the Numerals plugin helped me a lot when designing a scale model.
And the Markdown note:
# Unit definition
## Scaled base unit
```math
scale = 35
createUnit('ldu', unit(scale * 400,'um'), {override:true})
```
## Define units based on LDU
```math
createUnit({stud:'20 ldu'}, {override: true})
createUnit({brick:'24 ldu'}, {override: true})
createUnit({plate: '8 ldu'}, {override: true})
createUnit({'plastic': '4 ldu'}, {override: true})
```
# Test
```math
1 stud to mm
```
```math
3m to stud
2m to stud
1m to stud
```