You have invested
X in a stock which now has a value of
0.3X. You want to buy more, say
Y, such that your overall loss is only 10%, which means that
0.3X + Y = 0.9(X+Y). To solve for Y:
0.3X + Y = 0.9(X+Y)
0.3X + Y = 0.9X + 0.9Y
Y - 0.9Y = 0.9X - 0.3X
0.1Y = 0.6X
Y = 6X
So you would have to invest 6 times what you originally invested (or 18 times what it's worth now) to drop your loss from 70% to 10%.
Note, however, that you haven't actually gained anything. Your overall return has not changed if you include what you plan to invest. It just makes that stock look better, while drastically increasing your exposure to that stock. Trying to increase the "performance" of that stock is partly a "sunk cost" fallacy. You cannot change the fact that your initial investment went down 70%.
Another way to think about it is: if you did not own that stock at all, would you buy some? How is that different from what you're proposing?