computing shunt resistence for a microammeter
The calculation is correct.

If the micrommeter has a full-scale deflection at 100 microamperes, across an internal (coil) resistance of 1 kilo Ohm, then it deflects fully when a voltage of 0.1 volts develops across it. 

To generate that voltage against a 1 Ampere current, the resistance (shunt) required is 0.1 Ohms. The parallel resistance of 1 kOhm is negligible in this situation due to a 4 order of magnitude difference between them.

For lower current measurement, such that the shunt resistance would be within an order or two of magnitude of the coil resistance, the parallel resistance network comprising of the shunt and the coil must be taken into account.