The short answer is there isn't a short answer. How much alcohol directly affects your BG is hugely variable, even on the same drink. Sometimes it'll impede your BG more one day than it will another, even if you do everything exactly the same.
Like everything else with diabetes, there are no solid equations you can rely on. The best advice I can give you is to drink in a controlled environment (ie. at home) and then test hourly to see what it does to your BG. Bear in mind that alcohol doesn't have the glucose-reductive effect that a lot of people seem to think it does. Drinking alcohol doesn't automatically lower your BG. In drinks such as beer it'll actually raise your BG (sometimes quite significantly), and then several hours later, the alcohol starts to block the release of glucose from the liver, which is what causes the apparent drop in blood glucose.
Using sugary mixers is not a viable way of preventing this sort of alcoholic effect. What will happen is that the sugar from the mixer will get into your blood stream very quickly and significantly elevating your BG. You'll spend the whole time peeing and probably drinking even more booze. Then, several hours later, the reductive effect will start to kick in. This probably won't drop you enough to get you back to a safe level if you've been drinking sugary mixers, and even if it does, you'll have spent several hours running high with all that sugar scraping the **** out of your cells. Not a good option.
With mixers, your best bet is to stick to diet. When you get a little more 'pro' at this (god don't I sound arrogant
) then you can start playing a little more. For instance, when I hit a club I usually burn off a fair bit of glucose dancing. Most of the night, I'll either be on the lager or the spirits with diet mixers, but every now and then I might have a spirit with a regular mixer. This helps maintain my BG.
However, the golden, golden rule with booze is to eat afterwards. This is how you counteract the effect of the alcohol. Pick something that'll release glucose slowly into your blood over a long period of time - brown bread, crisps, brown rice. If you can stomach it, a kebab works well because the fat emulsifies the booze and delays the carb absorption from the naan slightly, meaning it's ideal for preventing BG going low (if not exactly ideal for your health overall).
Alcohol and Mixers 
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