Been thinking about your question for a while now Wedge and I'm just not sure that it will work with just a 20ema. It would certainly be safe as by only entering once the SP crossed above the 20 and exited when the SP was heading back to cross below it would certainly protect your capital and you'd sleep well having those trades open.
In fact it might be a master stroke for those who swing trade. Toss away any other indicator and just buy when the SP crosses above 20 and enjoy the ride until it crosses back. Then go sailing until it reverses again. Rinse and repeat.
Here's what I'm talking about.
I use an 8 / 20 ema combination and yesterday we finally saw the SP cross above the yellow 8ema on solid new volume. Now need to pause here as we need to take into account that the previous day had closed below the lower bollie band by a long way so it's always on the cards that there is going to be a bounce after that. Now back to those green candles, I think that the .083 entry was valid considering the previous day and also how the 5 and 15 min charts were rising strongly. So it heads off to the pink 20ema, gets there and stalls which happens quite often and one of the proven points in why I use a 20. When it failed to close above it the third time, that was the close signal (which I didn't take in this instance).
Point to all this Wedge is that if you were only using a 20ema you would have missed all the fun below but I'm warming more and more to your idea.
PS, I was very lucky to get out at .084 this morning to protect my capital. (trading Gods were looking over me)
Hope the above makes sense, please ask if it doesn't.
Cheers
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