It’s very hard to estimate if the concentration of the store-bought chamomile extract is exactly similar to the one you’d make yourself but as a rule of thumb it should be the same measurement. I’d recommend doing a test like that and monitoring how your scalp feels after the shampoo. You can’t really overdo chamomile extract unless the store-bought one you got contains irritant ingredients like alcohol or harsh preservatives. But otherwise, you’re good to go!