Writing a custom module for rules

This is difficult to explain so I've attached a schematic. I have to create a rule that updates raw material inventory when a product moves into production. There are several tables (content types) involved. The rule involves a couple db_queries with some looping and simple calculations until a field is updated.

Any suggestions would be appreciated. I am aware I'll probably need to write a custom module, but I haven't found any that are similar to my situation using rule_hooks. I am using rules 1/drupal 6.