Keep fajitas simple. I never measure the mix. I use citrus salt (do a search) black pepper, onion powder, garlic, cumin limejuice and a shot of tequila. Don't marinade too long the acid makes quick work of a flank steak. Make enough marinade to be able to brush or spray on your vegetables while cooking.
Trimmed Skirt steak, 1/2 cup - Allegro Hickory Smoke Flavored Marinade, 1/2 - cup cooking oil, 1 chopped Jalepino pepper. Marinade overnight. Sprinkle heavy black pepper when you cook.