In working overtime hours it is better to set a target number of production Qty rather than be stick to the settled overtime hours?

yes, but something will be important that the production quantity given at the same levels of product quality without any losses

It are good if the result between production and cost are balanced or best. That is a solution if you are late on your deadline and the customer satisficaiton. But if you make of hours overtimes for make hours overtimes, your bring damage to your firm, but also to you. Because you increase its charges.

Yes it is correct but I always prefer making a strong connection between (Target Number of production) and Quality.. That is the optimum benefit of target 

yes in some places its better

It's always better to set up targets in order to achieve better productivity results.

Its depends on how is your customers demand or behave. It is better to have a standard time given targets, but some time have to do a extra. Also have to consider company profitability.