May I ask why you prefer to mix up a spray rather than dusting corners and crevasses with the boric acid? Just wondering.

Mix up one part of the boric acid to ten parts water.

Be sure to avoid inhalation of the dust, whether making your spray solution or using it as a powder.

To effectively control pests like Siverfish you need to understand what they need to exist and that is a warm, moist environment. So the first step in control is to change that environment, if possible. Since mixing the Boric Acid, a potentially toxic substance for humans as well as insects, with water (most sources I've found say with boiling water) means increasing, albeit temporarily, the moisture level, I would opt to just use a dust providing I had no children or pets around.
Where I have seen Siverfish, in homes, simply reducing the moisture level has been sufficient to control these wee buggers. Perhaps this link might be of some help.

Here is a link that might be useful: Silverfish