Use a spawn area and choose the desired radius. then forge around it until the blocks or walls are perfect with the spawn area. That's how I'm doing mine on Kinetica.
An excellent example of what PRS is saying is Drago's tutorial on forging fantastically curved walls, over at the GoO.
yea unfortunately you have to be logged in to that forum to see that thread :S but imo the best method is using a respawn area or king of the hill marker as a guideline and then just do a lost of interlockingz around it..
I will try out the spawn area method. I'll also take a gravity hammer and beat the hell out of a block for a few hours lol. EDIT: Thanks for the help you all! I was able to finish my 28 large block circle! I also used walls to help guide the blocks that I was placing down. Thanks again! ~Plasmy Mods can go ahead and lock, question answered!