Doubt on long term investing

They usually say buy on dip. How is this dip calculated? Say one stock falls 10% in 1 month or 2 months. Now is it the dip? Is there any specific value of dip you follow? In percentage terms I try to buy only when the stock has eroded 10% from my buy value… not 10% market fall… because of this most of the time I don’t buy the stock and the value rises after… so like how to go about it. The thing is, most of my portfolio is like >30% even at crashes so I never get the chance to add Say now Asian paints tanked 30% recently and I was looking to buy it for 3 years and brought it today only