Much of the data we have determining the normal values are from glucose tolerance tests(because people's diets are so varied, it would be difficult to just pick one diet and use that as standard).
Any post-eating glucose should be less than 140, that is completely normal. Values between 141-199 are considered pre-diabetes.
The lower limits and rate of drop are not well defined. In practice, some people have post-prandial glucose levels down to the 60's and this can still be normal. But post-eating values of less than 50 should be evaluated by a specialist.
The rate of drop depends on what was eaten and how brisk the insulin response.
Sorry some of this couldn't be more specific. Hope this helps a bit.