Have you considered putting the switch driving the solenoid
before the step-up converter? That way you don't really care how much current draw occurs while the thing is off, you'll have the entire step-up converter shut down. You can get converters with an "enable" input pin, which makes it even easier. (Ed: repeating what Simon says).
Regardless, it's reasonably straightforward to figure out what your losses will be; just plug the current draw of the output voltage divider into the formulae and find out?
Don't use that particular crusty chip unless you use an external switch. I tried and just made a really hot thing that failed, the problem is the internal switch has such a voltage drop that in boost mode because more current goes in that comes out it's really bad on efficiency.
Not to be blunt, but did you do calculations behind the scenes for that particular chip in the OP's particular use case, or did you just criticize a perfectly serviceable chip because
you once attempted to use it in an inappropriate use-case?