2cents;
For design, nothing beats looking at valve's maps, the distances between spawn/capture point/checkpoint is insanely well balanced.
Another thing to keep in mind is the amount of entrances/exits each area has.
And further down the line an additional tweaksetting is where you place your ammo & health kits, might be something to think about allready.
And like previously mentioned, nothing beats working something out on paper first.
And finally keep checking it with different classes, something that feels good with a scout (distancewise) might be horrid with a heavy.
example top view of gravelpit with objectives and spawn rooms highlighted.