You should always determine your stop loss before you enter the trade. Once you determine where to place your stop you calculate how many pips large the stop is. Yes add your spread. Pretend 30 pip stop including spread.
The important reason for knowing your stop ahead of time is for POSITION SIZING.
I read somewhere on here that you guys like to risk 1% per trade. I do 3, but 1 is good.
Assume $10,000 account. 10,000 * .01 = $100
So for this trade you are risking 1% aka $100.
This is how much equity you would lose if you were stopped out for the full 30 pips. Now, for the position sizing.
You take the $100 and divide it by the number of pips in your stop.
100 / 30 = 3.33 lots
But, you’re not done yet. You then need to divide that 3.333 value by the pip value of the currency pair you are trading. This information of pip value can be obtained from your broker software, or website.
So for example, let’s pretend we are trading USD/JPY.
The pip value of usd/jpy is .8223 for example. So,
3.333/.8223 = 4.0532
You position size is 4.0532 lots.
At 4.0532 lots, you would lose 1% ($100) after a 30 pip stop out.