pity that the P4F/PTE website shows me as logged off after I click a link from the P4F forums. Even their website is buggy, let alone the game, that seems to have more and more bugs with each further update.
There's a upside-down "T" that shows where the bomb will land (around the point where the lines meet). Since the planes drop two bombs, you can tighten the bomb cluster by diving toward your target at an angle.
you can also fly as close to your target's ground as possible, deploy your bombs and then gain back altitude. Pretty risky but it works. (at least it did for me)