Return Scrolls in Towers

In Orbis tower, you get returned to Orbis if you use a return scroll on the 8-20th floors. (The floors with the wing emblem on your map.)

You get returned to El Nath if you use a scroll on the 1-7th floors. (Floors with the snowflake emblem.)

I don't know where you go if you use a scroll in B1 or B2. Same applies to Eos Tower.

I also assume that for Helios Tower, the library and 2nd floor will return you to KFT, and 99-100th returns you to Ludi.

B1-B2 will take you to El Nath.

Eos Tower, it starts at the place where 50th floor would be (since 50th floor is part of a large drop) That map goes to Omega Sector i think, and the one above it goes to Ludi. Basically, if you use 2 eos tower scrolls, you can use a return scroll to get to your final destination faster, and it will increase the amount of time you can go between repeating the Tower Scroll quests (since it now only takes you 2 scrolls to go up or down, as opposed to 3 before.)

Your assumption about Helios is right.