Any blood donors out there?

Hi,

I was dx 2 months ago and am following low carb diet, lots of exercise and no medication. I'm due to give blood at the end of the month, and I wondered if anyone had any advice on what to eat before I go, and what bg level I should aim for. I have promised myself a packet of crisps afterwards, but I don't know if I will be able to actually be 'naughty' enough to actually eat them!

Thanks. I'm just a bit nervous but I'll make sure I eat and drink as per normal.