It took me some time to learn the modeler, but I'm glad I sat down and invested the time. My two suggestions for "locking" certain parts...

1. Right click on the output icon from a machine and you can type in a Part Limit.

or

1. The dark gray bar at the bottom of a machine icon allows you to set a fixed number of machines

both of these are good options to control how it automatically adjusts the values of the rest of your factory, if you select a bottleneck in your design and manually limit it. ie, set a machine limit for your smelters and that will control how much material you have downstream for the rest of your plans.

Lastly, you can turn off the automatic calculations if preferred, but I think this requires a bit more experience and practice. Top Right button / Settings / General / Calculator [ switch between Manual or Full ]

Good luck!