Is the tubing hard or soft? Because that will make a difference.
My Mango has soft latex tubing, and I've found what works best for me is about 1.5-2" tube length from the back of the packer. Since my goal is to ultimately end up at the urinal, I found that that length of tubing gives me room to reach in and position it, and is flexible to lift the dick up and flip the top half over my shorts and fly and aim. With hard tubing or shorter tubing, I'd have to pull my pants/briefs down farther in order for the stream to aim at the toilet.
You can try it at the length you have and see what works for you. It's going to take you a good few weeks to really get the hang of it. I felt confident after 3 days, then embarrassed myself quite a few times (once or twice at work even!) by having accidents.
Jay