Basically, you are finding the maximum and minimum values of refractive indeces. Typically you need to know the optical axis of the well aligned sample. This can be done by applied a magnetic field (apply a electric field works but has a lot of complexions, so don't recommended if you are not experienced), then just measure those using refractory just as you do with isotropic materials, only you need to measure twice instead of once.

Also, if you made two measurements and know the angles between the optical axis and the direcitons you are measuring, there is a formula that leads to what you wanted.

By the way, make sure it's uniaxial. There seems to be evidence that biaxial liquid crystals exist (although this not well accepted by the community right now.)

thank you sir 4r ur rply..