There is never a specific amount to describe what "Dip" is when it comes to Bitcoin, because the All Time High you see today could be another deep in either a months or years time, which is one beauty about Bitcoin, of which due to it's usefulness, gives room for more demand, hence, result to an increase in price value. So on that note, the best advice I can give is for people to keep buying the amount of Bitcoin they know they can always afford to buy without stress of financial instability, while having the mindset to hold for a stipulated long period of time
Not any specific correct answer about dip price of bitcoin because after lower or dip always has other dip price, today bitcoin drop under $63,400 although OP said dip price around $64k but always has possibility with other dip price.
But take risk by investing or purchasing bitcoin after dropping around $64k seems good decision for every one who braveness take risk reinvesting right now after most lower price of bitcoin in last several weeks.
Keep manage well with investing your money in bitcoin by spending around 40% every dip price and buy back later with potential for bitcoin making other dip price or going down under $64k. I use this kinds of investment way when investing in bitcoin keep save capital around 50% preparation if bitcoin get decreasing suddenly.