Do while loop in excel

Hi, I am a stock market speculator and i developed a trading system in excel, but i am stuck in one place which is need to be resolved, unless i cant go ahead in it.

I calculate % difference in between High and low of a stock price, as soon as it reaches my required % it fills the cell with "TRADE".

As soon as "TRADE" is in 1st cell, second cell calculate the price through some formula(which need not be explained here,also not necessary), which uses
High, Low and another cell value which changes every second(which is called LTP).

So what i required is as soon as it calculates the value for second cell, it do it only once and stops calculations even LTP keep changes.

In any programming launguage a do while loop can do it, which says keep doing the calculation until another given condition is not fulfilled.

So in my requirement i need to keep checking the calculation until the second cell is blank, once it gets a value, the calculation should stop there.

I hope i explain it properly and someone will be able to help me. But if you need to ask me anything you can contact me
Krishan sharma