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Also important factor (in case of daily ema21/ema10 it is not as much important as mentioned above but still should be considered) is the estimated price of executed order.
I don't know what price Goomboo is using but it is depended on market liquidity and volume of your trading assets, if assets aren't huge and market liquidity isn't tiny you can assume you will buy/sell on the next period vwap price.
Some tools like cryptotrader.org overestimate the price a bit because they are using current period price (the period on which signal was generated). That is not strictly correct because you can calculate the signal only when the period is closed already, and when it is closed already you shouldn't assume you will buy/sell on it's average price, the fastest possible is next period Open price, still quite unrealistic.
If you are going to put money on some strategy I would advise to backtest it not by external tool, or at least validate the backtest results by your own, it can be achieved even in spreadsheet.