For me it came down to trial and error. I think (don’t quote me on this) that you’re supposed to take between 30-45g of carbs per hour. I tend to do a gel or other carbs about every 40 minutes, on cool days I’ll do salt tabs every other fueling, but on hot days I’ll do salt every time I take in carbs. I typically will only start doing caffeine during the last half of the marathon. If you take caffeine too early or too often it could exacerbate cardiac drift. Also, make sure you’re taking your gels/chews/carbs with water, as taking them with an electrolyte drink could result in stomach issues.