Also, when you put the jump drive into the ship design, it will increase the mass by more than the mass of the jump drive. This is because of armor - the ship is bigger, so it takes more armor to cover it to the specified depth. This is important to think about if the jump drive isn't yet researched (and so doesn't show up on the F5 screen). For example, if you want to end up with a 120 HS (hull space) ship and your jump drive will be 30 HS, you should not design a 90 HS ship and assume it will be 120 HS after adding the jump engine. Instead, you should add 30 HS of "junk" (fuel tanks, extra engines, etc) to the design while you're experimenting to substitute for the 30 HS the jump drive will take up.
John