How do you perform a MYSQL After Update Trigger Insert Loop?
You can restructure your query

INSERT INTO raw_materials_stock 
  (batch_id,raw_material_id,use_date,quantity) 
    SELECT 
       new.batch_id, 
       b.raw_material_id, 
       now(), 
       ((new.volume/100)*b.percent)*-1 
    from base_formula b where b.base_id=new.base_id;