A guideline that I started using in the 70's and still use today:
Too thin/soft/flexible..it is likely to curl around,kink off the flow,not go any farther in.
Too thick/stiff/hard.. it will not easily follow the contours of the colon and stop,a higher risk of perforation.
Lubricate it well,relax and take your time. Insert it slowly and steadily, backing it out a bit and turning then inserting a little further as needed. Keeping a low as possible flow of water until the desired depth has helped me also.
That being said, I personally like colon tubes in the 48inch or longer length, 36-42Fr size. I use them for deeper cleaning and/or play after I have done 2 or 3 enemas to remove waste that would inhibit insertion and flow. There have been many times in the past, that, after doing initial clean outs, I have spent extended time laying in a shower tub with a bag rigged to constantly fill, with the flow to the colon tube set comfortably. Each time I get full enough to release, I expel around the tube(much in the same manner of the Libbe system)and insert it a little deeper. What little waste there is left readily goes down the drain because it is so small. As a result, I have managed to get the colon tube about halfway into my transverse colon. Once there, I have left it in that location and have had a wonderful time and results, letting the water slowly/gently massage the colon and wash away lodged material in addition to being erotically stimulated by safely filling without worry of making a mess.
I hope you find something useful in what I personally do when using colon tubes, if not, then set it aside, no problem. 😃
G.N.