This is the recipe that I made for my sheep when I had them...
2 cups apple cider vinegar
1 tablespoon mineral or olive oil
1/2 or 1 tablespoon dish soap (preferably one with lemongrass, peppermint etc)
And 40-50 drops of any essential oils that help repel bugs...you can mix and match them
Lemongrass, peppermint, geranium, eucalyptus, cedarwood, clove, citronella, lavender, tea tree, rosemary, pine, thyme and cinnamon.
Add water to fill the rest of the bottle...it fills a 16oz sprayer. @Baymule recently told me about Mexican Vanilla for repelling bugs and it works really well!
You could actually replace the water with it and it would be extra strength bug spray...plus it'd smell amazing!
I got my Mexican vanilla from Amazon...a 2 pack of 24oz bottles for $18