How high does a jet have to be to hear its radio call?
The formula you're using is correct, but don't forget your own altitude.

$$d = 1,23 × (\sqrt{h_1} + \sqrt{h_2})$$

So in your case:

$$123 = 1.23 × (\sqrt x + \sqrt{1000})$$
$$100 = \sqrt x + 31.623$$
$$\sqrt x = 68.377$$
$$x = 4675\ \mathrm{ft}$$