To correctly set the power regulator to the 70 mark (indicated by the red arrow on the display) you'll need to play around with how each switch affects the needle.
The door is locked, so read the notes on the ground near the door to figure out how to open it. She'll appear on the other side of the wall in an even more ghastly torture chamber. Proceed to the next room - it's a bit sordid - and find the corner with a small hole where you can give Rebecca a boost. There's an Ink Ribbon next to the type writer, so feel free to save your progress if you're looking for a good spot. Once you've reached the Basement Safe Room examine the Green and Blue Herbs on the floor, take the Basement Map on the wall, and you might as well check out the Inmate List on the table (though it really has no pertinent information). Just run your characters straight down the hall, then take a sharp left turn (stick close to the corner) and reach the door and the end of the hall. The best way to make it is to split the team up, and run in one at a time until you reach the end. They're not worth killing, but you also want to make sure you don't dabble around in the area too long - if they manage to spit their venom on you you'll be poisoned until you take some Blue Herbage. As a word of caution, the hall that leads to the Basement is lined with giant spiders.