Try turning on tempory axes and measuring using that. If you are working with assembly cuts, tempory axes will not be displayed if you are using SW2003. (it is a bug that is suppose to be fixed in SP04). If you are using SW2001+, the tempory axes will not be displayed but if you put your mouse cursor where you think it should be, it will highlight. Otherwise for assemblies you can use the dimension tool and select the circle and then the edge. As long as they are on the same plane you will get a valid dimension.