How does a company determine the dollar amount for obsolete inventory?

It seems that some companies assign a percentage to an inventory category according to its sales trend - e.g. 50% to non-moving inventory and 25% to slowly-moving inventory. Is this the MOST COMMON method?

How about write off everything that is over 2 years old?

The most common method is to put the
Obsolete stock in the dust bin or
Better still ship it to the tip.
You can determine exactly and literally what you write off.