If you trade a range of instruments, then you should be aware that for a given risk amount the volume (lots) can vary from 0.01 up into 100+ in value. This a a factor of over 10,000 in required lot size depending on the instrument you are trading. Why would this be a problem you may ask?
Well, if you get your calculations wrong then that 100 GBP risk you believe you are taking could actually be 1,000,000 GBP and you won't even know it until your order is triggered and your broker issues a margin call because your account has just been destroyed!!!
If you are comfortable with calculating trade volume using spreadsheets with info from your broker, then provided you are careful and using the correct data, then you will be fine. But because volume varies by such a large factor (10,000+) depending on the instrument you are trading, any error can result in unexpectedly high risk, which is hidden under the guise of 'volume' ... rather than an easily understood risk amount in your account currency. So you'd better be sure of what you are doing!
Using volume to fill out a new order ticket is not a good way to trade as it hides your risk, and it typically means you have to resort to spreadsheets and broker supplied trade specifications in order to work out what volume is required. This is slow and error prone and is not how most of us actually trade (i.e. best practice is to use % risk based on account equity with 1% being typical). Using volume is a dangerous distraction.
But this is not the end of it ... there is another danger if you are using an EA that calculates risk amount based on symbol properties available via MQL ... yes it is possible to automate volume calculation, so you can work the way you want (i.e. using entry price, stoploss and risk %) ... I tried this using three possible methods to calculate volume (lots) required (see above link). What I found was that the data coming from my broker (lotsize, point, ticksize and tickvalue) and made available on MT4 was often incorrect resulting in my risk exposure being 100 times larger than expected on some instruments. I tried another broker and found issues again. What this means is that you cannot trust the symbol data coming from your broker!
It is the broker's responsibility to set up the trading platform correctly, but I found that this cannot be relied upon. And if you tell them it is incorrect they will still not fix it. I believe this is because they perform all their volume calculations manually on spreadsheets using data they have compiled on spreadsheets and listed on their website for clients. The effects of incorrect hidden symbol properties that are available via MQL are therefore invisible to them.
To me all these hidden traps that can destroy your trading account are unacceptable. Forcing traders to use volume in order to enter a new trade is a dangerous way to manage risk as it is hidden. It is also not the way traders actually trade ... we don't care about volume as all trades are based on risk amount as percentage of account equity.
I contacted MetaQuotes to point out the issues mentioned above and suggested that they consider adding a risk amount based order entry screen as an option for those that want to avoid complex, error prone, slow, volume calculations, but so far they are not interested. I don't think they realise what a big problem this actually is ... particularly for new traders. Maybe if we all shout loud enough MetaQuotes will provide a solution that allows us to be certain of exactly how much we are risking on each trade so that there are no unexpected surprises. So please feel free to comment...