Visible Light Transmission or just light transmission, whatever they want to call it is basically how much of the light it lets through from the other side.
with films you should also find the light reflection number, this will indicate if it would actually work as a mirror or just a shiny metal pot with somewhat of a reflection in it…
Anyhow, the higher number the more you will see of the object behind. 70% is a big no, you will pretty much see everything, even if it’s turned off.
I would suggest 15% or 20%, if the room you are going to place the mirror is bright/really bright, I would go with the higher number. If it’s darker I would go with 15%.
And Have you actually asked any glass businesses if they can order it for you? Most major glass producers have a two-way mirror option.
The room would be a bedroom so average brightness. I have emailed about 10 custom glass and window companies and waiting to here back. The one that did get back to me is $200+ for a 24x36" so its likely out of my budget.