Bilberry is very strong. Click link below and you can download Hic's notes on all FA flavors free. Great notes on bilberry for you there. He also has a great list of recipes for sale if you are interested.
Oh to answer your ? I usually use it around .5% in a recipe. I have not tried it standalone, but it is one of FA's stronger flavors. To me it needs to be sweetened with other flavors, but that is just my opinion.
I'm a .5%er kinda guy myself. It's like FA raspberry and needs to steep a few days to develop.
Try the ole fruit test base.
Bilberry(FA) .5%
BASE
Fresh Cream(FA) 2%
Vanilla Classc(FA) .5%(FA Vanilla bourbon will work)
I know I'm an odd duck I like my billberry strong. I Like to chew on the flavor and I think my ability to vape it at a high percentage must be odd. But I liked it at billberry FA 3% and joy FA 2%. The billberry just was a zingy flavor in the first 3 days and mellowed after that.
It may be worth wilded to start at 1-2% first if mixed with joy FA. But that's just me. And probably is a simple cream recipie the 0.5%-1.5% should be a good place to start too.
There is a billberry thread in somewhere in the DIY section. Just search for billberry using the search function at the top of the page.