Actually, You CAN judge where your shells will land. when you aim the crosshairs at a point, you get several numbers... on the left, you get range, and time to impact for that point, on the right, you get the current elevation of your gun, and the "REL" number tells you what elevation you SHOULD use to hit the point under your crosshairs.... it's already easy enough.
Now, with that said, there is some variation as to where rounds will impact, and this is intentional in the game.
The HUD version of the targeting computer has a MINIMUM range that it will calculate elevation and TTI numbers though. Below this minimum, it's pretty much just guess work as far as I know..
Self propelled artillery really wasn't meant for direct fire though, even though it is more than capable.
SPA's were designed to fill a specific role, which is to provide fast, mobile indirect fire on fixed/mobile enemy positions with a small crew. SPA's can stop, setup, fire, break down and be ready to move again in just a matter of a few minutes, and catch back up to their unit.
With field artillery like the M109 or the M777, they can still do this, but it takes much longer (15 - 30 minutes) and requires a larger crew per gun.
I've attached an image above, that might help you, and anyone else having the same trouble. Hope it helps!
UPDATE: On the image, the number at the top of the hud "015" on my image I had listed as the direction of the turret. But I'm not sure if this is accurate. On my game, the number never changes, no matter what direction the turret is facing.