I've got a trick for working out if you've got enough weight. Put your weight in, then dry fit your major parts, holding them with masking tape. Balance the model on something like a pencil or brush, where the rear undercarriage sits. If it falls backwards, keep adding weight until it tips forwards, I usually then add a little bit more for good measure, to account for those little parts that might just tip the balance back again. Doesn't have to be in the nose, anything in front of the tipping point will work, although the further from it the better.
