Finally, now(!) I am in picture. Thanks for taking your time, it does make a difference when I understand internals.
Believe or not until now I thought (z)xtool simply scan first stream and.. that's it - use same info for *all* rest of data lol
. Reason I thought so was that game packs are likely using single specific level and that could in theory be enough(to pack the game, for which zxtools were designed).
Since that is not the case and your approach is clearly even much better than mine we can rest this case. Thanks again.