Hello Ian,
You can set the default installation path on the
Install Parameters page -->
Application Folder field.

- app folder.png (52.28 KiB) Viewed 11923 times
To check a registry entry, you can use a
Search.
1. Go to the
Search page.
2. Click on the
New Search.
3. Click on the
Add Search Location --> Registry.
4. Configure your search to retrieve the desired key value:

- search registry.png (105.73 KiB) Viewed 11923 times
5. Go to the
Custom Actions page.
6. Add a
Set installer property custom action with sequence:
Property: APPDIR
Value: [RESULT_PROPERTY]
7. Hold the
SHIFT key and
drag it after the
Searches action under the
Wizard Dialogs Stage.
8. Add the
RESULT_PROPERTY condition.

- custom action.png (74.09 KiB) Viewed 11923 times
After that, if the registry key exists, its value will be used for the installation path, otherwise the default value will be used.
Attached you can find a sample project.
Hope this helps!
Best regards,
Liviu