Rosemount GWR Issue

Hello Everybody!
I have a Rosemount 5300 Series Coaxial probe guided wave radar (HTHP type). I wanted to install it in place of a displacer in a displacer chamber. The original length of the probe assembly (Flange bottom to probe tip) was 762 mm. I reduced it to 693 mm to match my chamber's height. The procedure for cutting the coaxial type probe is provided in Rosemount reference manual.
Before cutting away the extra length the transmitter was indicating the actual physical length of probe dipped in the liquid level, quite precisely. After cutting the probe to desired length I configured the new "probe length" and "Tank height" parameters in the transmitter. However, there is a constant error of about 10 to 15 mm in the indicated level.
Also there is a dead zone created of about 20 mm at the tip of the probe. In the dead zone the level is shown 0 mm regardless of the actual level.
Anybody having experience with this type of GWR please guide me about what other configurations I have to change to remove these errors?
Thank You