Calculating Tolerances on a Part

Hello everyone.
I have a question about calculating and/or estimating tolerances of dimesions of a [B]non-cylindrical part[/B].
for example, we see in a drawing that a certain length dimension is this : 50[SUP]+0.25-0.5[/SUP] , my question is:
How do engineers calculate the tolerance value ?

I have been looking in the internet and in books but with no results.
If there were any documents or books that treat this question thoroughly I'd appreciate it if you provided me with their name.
PS: My question is about [B]non-cylindrical parts only[/B]. Shafts and holes already have an ISO dimensionning system.

Thanks in advance

There are two approaches in mechanical tolerance assignment...

1) Tolerance for function. This is the mechanical tolerance required to achieve the required functionality.
2) Tolerance for Manufacture-ability. This is the mechanical tolerance assigned based on the specific manufacturing tolerance available for a feature or part.

See: [B][URL="http://www.engineersedge.com/catalog/product_info.php/products_id/28"]Engineering Design for Manufacturing and Assembly[/URL][/B]

It depends on function of the part. And the tolerance is stated clearly next to the dimension and you have to follow it. Why do you looking for something else? Remember that not all Cylindrical part follow ISO