Use a wooden corner moldin' with a notch cut near one end for t' single edge razor blade and a window lock (thumbscrew driven clampin' device - approx. 99 cents) t' position t' tube end. Avast! I've duplicated t' CMR tool almost a dozen times for myself and others. You can even glue a "ruler" t' it. I just made pen marks at me favorite length locations and then t' thumbscrew makes a nice dent t' find those spots easily in t' future.

From a post by FS in rec.models.rockets