Generally speaking a share price will drop after the stock goes ex-dividend, by an amount roughly equal to the dividend. Note this is not the payment date, but the date on which the shares trade without entitlement to the dividend.
So when FMG goes ex-dividend (which is 1st March, this Thursday), you would expect the share price to drop roughly 11c. In reality it won't drop exactly 11c, because of the regular market fluctuations.
As a retail investor, you will also get franking credits which is something to consider. FMG's dividends are fully franked, so the value to you of the dividend is actually about 15.7c in gross dividends when you include franking credits. Institutions and big investors don't generally get franking credits, and they are the big boys who dominate the trading action, so the price drop generally reflects the unfranked amount (11c) instead of the franked amount.
The other thing you need to consider is capital gains tax, which you will probably owe on the $800 you make if you sell now.
The effect of these two tax events (franking credits and capital gains tax) mean that generally speaking, a retail investor who wanted to sell shares close to ex-div date would be better off holding until the ex-div date and selling after, because the tax on the dividend is less (due to the franking credits) than the tax on the capital gain.
But like you said, there is no easy exact answer because who knows what the share price will do between now and Thursday. It might drop 10% overnight in which case the difference between the two strategies above is negligible compared to the 52c you lose per share from the drop.
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