Hi parsa79,
for Siemens in ideal case shoud be set: HOLTHLVDL less than RXLEVMIN (1 or 2 dB).
If you have colocated cells (900/1800) than you could set for 1800 band (capacity layer) RXLEVMIN=16, HOLTHLVDL=15 and HOLTHLVUL=5. For 900 band (coverage layer) you could to set lower value.
BR