If you think it is too high only buy a small amount with a plan to buy more if it goes lower. This way you will at least have some if it goes higher.
I don't think this is a problem of the high and low calculation when selling when we buy when it is high whether later when we sell it will exceed the price of the purchase, because everyone knows the price of bitcoin when it falls does not take a long time but if it goes up it's not playing our time to wait, so in essence our calculations do have to be right don't just go away.