I use 100mg/ml concentrate for the reason of mathmatical simplicity. If I'm making a 10ml batch, and I want 7mg/ml on final... I put .7ml concentrate in the bottom of the bottle, and add the rest of the stuff.
If I'm making a 20ml batch, i double it.
Keep in mind that this must be a ZERO FAIL process. failure to make accurate calculations/measurements can result in making someone violently ill/dead. If you are not confident in your mathematical skills, perhaps you should stick to commercial juice.