You can use a ruler, but rulers are a cruel mistress - they're TOO straight. I tend to find that if I draw a tank with very straight lines then it looks too much like a technical drawing, less like a painting.
this is cool bro, i like the composition everything is layed out nicely. i really lik ehte lighting on the dropship that has landed and is unloading the dreadnaughts. if i may say i dont reall like how soft the rocks to the right of the tank are,maybe need more definition or contrast/shadows. but still nice pic duude.
Cool! I've never seen Dreadnaughts being dropped off before. At least not in a ship. Fantastic job all around. Especially the cloud of dust underneath the one flying away. It looks like it's going WOOSH!