50-60% or so i hear. I'm experimenting with it, but thats what i've read. I'm having a tough time keeping my levels steady, i have aquariums from when i first got into snakes X_X

While I don't have any experience with Sandboas I can tell you what I do with my ball pythons to help with shedding.  The racks I keep them in hold humidity pretty well, but when it comes time to shed I'd mist them at least once a day.  Also creating a humid hide can help, click here for more instructions for how.  I don't know the exact % of humidity that sandboas need, but by simply misting and providing a humid hide they should shed better.  Also if they are in a tank with a screen top then I would place something such as plexiglass over the screen to help hold in the humidity since it's leaving right out the top.

Hope that helps.

Hopefully the sand boas will use the humid hides, I just wanted to chime in and say mine have NEVER used it. I think those work better for terrestrial snakes than burrowing snakes.

Tiki.. how they gonna get oxygen? lol, just sayin cuz thats what i do have them in is glass tanks... ive heard of taking pcp pipe and depositing some water directly to the sand (and yes I do have my sand boas in SAND, dont wanna get into a substrate battle with anyone!) Sapphire u ever heard anything like that?

You obviously don't seal it to the top, most people leave it over the top with a little crack on one side.  Some people will cover the whole top, but open it once a day to allow fresh air in.

Gotcha! Hmmmm maybe Im just gonna need to get a couple humidifiers for the boa room...?

Resisting the urge to get in a debate about sand....*Sigh*

I mist the screen tops heavily inside, and then wet a towel and put it on 95% of the cage top screen. This bumps humidity right up and keeps it up until the next day, when the towel dries out. Rinse and repeat until sheds are whole. :) 

We removed the substrate in our cage and put in a wet towel for him to roll through. It worked great.

Yep, the sand is probably not helping at all with the shedding. It's dirty and dry. Otherwise, a good misting while they are opaque should help you out. Definitely cover part of the lid (3/4 is ideal) as all of your humidity is just escaping through the top. I am not a big fan of humid hides or assisting sheds over and over as you're not fixing the problem at hand, which is that the cage itself is too dry. Humidity isn't only necessary for ease of shedding but also respiratory health. 50-60% humidity is ideal for most boa and python species. I always keep my KSB's on Carefresh and they do really well on it. Good luck!