hi i have a DR F Protractor but it will not work on the sem 309 or any other sme come to think of it i have talked to Dr Chris Feickert who said it was SMEs fault and not his , you can use it for the pivot point but not the NUll points , or use it for the Null points and not the pivot point , so i was stuck , i was sent this web address that has the correct way to set the null points on sme arms and a great deal others ,

http://www7a.biglobe.ne.jp/~yosh/armdata.htm