How do you figure out black pipe length?

I have a tank that is connected to the outside world via two 1/4 inch black pipes, run through a concrete foundation.

The old tank needs to be replaced, but the new tank's bung holes are in different spots than the old tank.

Getting the first pipe to line up with the old pipe through the wall isn't very difficult, as I can attach approximate lengths of pipe and move the tank to line up.

It's the second pipe. Once the first pipe is connected, I can no longer move the tank again.

This second pipe comes vertically out of the bung on the tank, and goes into a 90 degree elbow. I then have to connect 3 or 4 inches of nipple to this elbow, pointing behind the tank. At the end, the pipe has to jog 90 degrees to the right, and connect with about 15 inches of pipe to a female thread on a union tied to the pipe that goes into another elbow pointing behind again, and then through the concrete.

Any advice on choosing the pieces to insure that everything lines up? How much pipe goes into female threaded openings on fittings so I know how much to add if I measure from union to elbow?

I have to have HD cut and thread these for me, as I don't have a threading machine.

Thanks

Rather than make that connection so critical I would consider a transition to a piece of flexible copper tubing or other alternate, approved materials.

But here are the thread engagements.

Thank you. Just what I needed.