Hi,
250-350 is the recommended, so your not far off the mark. What is the sanitiser in use? if your using tablets/granules these will increase your CH so you might not need to do much. You also need to test your source water to see what it is adding or taking away from your pool chemistry.
Calcium Chloride is used to raise CH and is readily available. You should use the Pool Math app to calculate how much to add (available via the menu)
Calcium Chloride should be spread across the deep end of the pool, be ready with a brush in case it settles on the bottom