OK, here's my take on your question......
You cannot convert a met viz to RVR (converted met viz or CMV) to a value lower than 800m. So your first example of night with HIALS enables the met viz to be converted by a factor of 2.
2 x 500m = 1000m CMV and you can make an approach.
If the met viz was less than 400m, i.e. by multiplying by 2 it comes to less than 800m CMV it is invalid.
Therefore a simple rule of thumb for a
night approach with HIALS requires a met viz of 400m to be converted to a CMV of 800m for all approaches requiring 550/600/650/700/750 or 800 RVR.
To answer your questions, you can make an approach in both of your scenarios.
By day with HIALS you multiply the met viz by 1.5 instead of 2.
Hope this helps (well, it's certainly my understanding, but I'm always keen to learn),
Sid
