How much cash do you keep reserve for dips

I’m guessing most keep have 10-20% cash on the side for dips like this. Do you have a rule of thumb for how much you would use for dips like this? It seems like most stocks are down a lot more than the indexes today so it seems kind of difficult to judge how much to allocate. E.g if indexes are down 0.5-1% but your stocks are down 3-5%, a further 1-2% downside on the index may or may not mean another 5-10% discount on your stocks. Do you guys go on impulse on DCA the dips?