Anyewo:
It will depend on the type of aircraft you are flying....but for the A340/330 I use the following:
Top of Descent: 4 x Altitude (making an allowance for airfield elevation)
High Speed Descent ( >250kts): 3 x distance to go - 4000ft
eg: with 50 track miles to run I should be at 3x50 = 15000ft - 4000ft = 11000ft
Normal Descent: (<250kts): 3 x distance to go - 2000ft
Of course in China you'll have to convert to metric at some point!!
There are many far more complicated formula around but I find this works well and and gives you 10Nm to slow up for the approach.