Like Preacher said, measure, remeasure, then sit back and take lots of time before you make the first cut. We've all at one time or another thought we had something measured right, we cut, then it's "OH S***!!!!" Metal is SO hard to add back in, but so easy to remove.
As for if you should take doors off or leave them on, it is really a personal preference..........we take em off so we can get around the top easier, then we cut the door window frames to fit like a jigsaw puzzle kinda. There are also two ways (at least) to make the top section big enough to fit back down on the lower section. (think pyramid. If you remove a section out of the middle of one the top is now too small to meet up with the bottom section) One way is to cut up the top and put metal in as gap fillers. The second way is to lay the windshield back and maybe tilt the windshield posts in just a tad so it all fits together. I like the second method. I think it takes less metal work and looks swoopier.
Masking tape is your friend too. You can run lines with it to mark off how much chop you are doing and then sit back and eyeball it. Finally, you will get this scared, sinking feeling when you actually cut off the top and lift it off. Everybody gets to that point but the feeling passes as you move ahead. And what's the worst that could happen? Roadsters are cool.
Don
If you tape it off like this you can visualize somewhat how your chop will look.
No turning back now. Notice how the section that will be removed from the windshield post is about an inch less than the back section? That is because as you lay the post back it uses up some of the height and if you cut it the same you would have to add metal back in.