Your tuning is mainly based on length of whip more so than length of bottom shaft. The reason it probably seems to tune better with a longer shaft is the center load and the whip tip is higher and further away from reflection and obstructions! Remember in the antenna world "Height is Might" If you change your shaft length get a new whip usually about 12.00 bucks. And PLEASE don't bend the whip to tune the antenna that is the ugliest way to do it!
Get a set of bolt cutters and cut it so it looks nice and professional....Cant stand it driving down the highway and look over and see Billy Big rigger with his Bent up white trash 75.00 Wilson 5000 antenna whip!