First of all you probably shouldn't be measuring yourself. It is hard to get an accurate measurment that way. When you measure, do not put your finger under the tape either. It should lay on the body and you should measure lay the tape at the top and bend back to get an accurate reading. For the most part girls that wear a size 6 who have come to me in the past generally measure about a size 10, size 8 a size 12.
When you take these up, be sure that you change the darts too they are recentered at size breaks.
We tend to wear our clothes a bit tight in the US and we use the built in ease to buy a size smaller...how many rump sprung skirts have you seen lately? The thing is that patterns all vary a bit by size Simplicity and McCalls runs fuller, Butterick usually runs slimmer and Vogue a little slimmer still. Butterick and Vogue patterns often fit the younger adult and even teen figures where the fuller figures seem to fit the Simplicity and Vogue better. On the back of the patterns, many times you'll find finished pattern measurements (which is a HUGE help) along with the measurments they recommend for each size. This is a great chance for you to compare and decide for yourself. At the same time, there are certain eases that almost have to be in some garments, more than you might realize...you need space to move to twist, bend your arm, etc. As you remove the extra in these patterns, be sure that you don't remove too much or the garment won't be comfortable.
Also, on the back of most patterns you'll find a description of the garment. This holds keys to how the designer visulized the garment and how much ease they have incorporated into the item. Here is a chart that may help you...http://www.butterick.com/tech/ease/ease.html
This chart happens to come from the back of the Butterick pattern book but most pattern books have them...along with a lot of other helpful information. These measurments are very accurate as to what kind of ease you should build into a garment to get "that type" of comfortable fit. These charts are also often included in clothing textbooks and are pretty handy to have around.