Guy
Another option which I have found works quite well is to put a very slight bend on the last cm of the dipstick. You only need a couple of degrees and this helps to stop the snagging...
But if you did want to shorten it my approach would be as follows. First make sure the oil is topped up correctly to the 'full' mark. Find a new shorter piece of tube, or trim the existing one to the desired length. Then shorten the dipstick a few cm at a time until it fits the shorter tube without fouling anything. Obviously as you get closer to the correct length reduce the amount you are shortening by, so as to avoid going too far! Once you have the stick at the correct length, simply punch a line across at the level shown, which you know is correct for the full mark, then put another one on for the low mark, referring to the original marks for the correct distance.
If I were to do it, that's how I'd go about it, or am I missing something?
But I'd try the bend thing first!
Tim