I ordered 70% dark chocolate from an online store but they sent me 99% chocolate. I've been trying to find measurements on the Internet on how to turn it into 70% dark chocolate, but most of the sites don't say a specific amount. As Christmas comes closer, I'm starting to panic. Any help would be much appreciated. 
If it's any help the precise conversion for any two concentrations is C1xV1 = C2xV2 where C = concentratiion and V = volume or weight. So, for 1 unit of 70% from 99%: 70 x 1 = 99 x V2 rearrange to make V2 the subject: V2 = 70/99 = 0.707 So, if you took .707 g or 0.707 oz of 99% and added it to something to make up to 1 g or 1 oz, technically you would have 70%. However, adding sugar or milk won't make you chocolate that will set like a block. For that you need to add cocoa butter and likely some sugar. How much sugar would depend on the style of block you aim to make. If you take your favourite brand of 70% chocolate and look at the dietary information on the back (assuming you are in a country with this information provided), then it will tell you how much of your 70% block is sugar, though if you are in the USA, you may need to do a little conversion from servings to percent. The popular brand Lindt, has, according to this website 12 g sugar per 40 g serving (4 squares) in their 70% block; so 12/40 = 30%, so almost all of the remaining 30% is sugar. I don't know how much of this is natural sugars present in the cocoa to start with. However, if it is any help, the same site has the 90% as 3 g sugar/40 g = 7.5%, so less than the 10% you might expect if the sugar makes up all the additional mass. 