To add to this, my pack "Holiday Lemmings '16" (which I still need to convert to the latest NeoLemmix version, because some levels are now impossible due to the deadly ceiling)... it only has 30 levels and the difficulty climbs quickly, but the first three levels start with X of everything to get you into the groove - however, each has replayability in the form of a talisman.
Level 1 - 20 of everything... easy enough starter level with some slightly tricky terrain to navigate. Comes off as a classic 20-of-everything level. Has a talisman that challenges you to complete it without any builders.
Level 2 - 10 of everything... starts with a deadly drop (and more than 10 lemmings). Completing it in under 1 minute nets a talisman.
Level 3 - 5 of everything... skills are more limited. Has two talismans: complete with 100% (difficult because there's a steel floor under the trapdoor, so stopping the lemmings from going to the left requires 3 Builders), and complete with no Bashers (requires the use of bombers and careful skill conservation).
Level 4 - 3 of everything... doesn't have a talisman, and barely counts as an X of everything level, but you have to use what you have so 3 lemmings can help each other reach the exit through a large steel obstruction.
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To mention another pack... I helped convert some levels in Revenge of the Lemmings to the Picnic rating which is largely Fun and Tame-like levels. A couple of levels that stand out is "Hunting Season" and "Singularity" because the intended solution in their harder versions is disguised in some way. The former, "Wish you had them?", has no Builders and it's not obvious how you get to the exit without them. For the latter, "Duality", the limited skills mean you can't just brute-force your way to the top with Builders, and uses, in my opinion, an absolutely beautiful sacrifice as part of the intended solution.
And this is just me... when I played the Fun levels and realised they repeated in later difficulties (as well as the first 13 Tricky levels), I started to wonder with anticipation when the next repeat will appear! In fact, the only non-special Fun level that doesn't repeat is level 3: "Tailor-made for Blockers", probably because it was actually tailor-made! I tried to follow the same philosophy with the Picnic levels in Revenge of the Lemmings with, at the time, only "Picnic 29" not repeating (although a harder version was made for it later).
Finally, with all these easyish levels in Picnic, you can just skip straight to Hootenanny if you don't want to play them.
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As mentioned before, one problem with difficulty is that, as a level designer, it's hard to accurately measure the difficulty of one's own level. I thought the difficulty of my level "Dropship 12" was on par with high Mayhem or Havoc, given it uses a variant of a famous Mayhem technique, but feedback from others suggest it is one of the hardest levels I have ever made. Granted, such overestimation or underestimation of difficulty is generally not too extreme... only one or two levels out.
Though not with Lemmings, I have used a similar system for rating difficulty in the form of stars. Usually rated from 1 to 5 stars (although 0 can easily be added to this), and "Flashing 5-star" for something insanely difficult. I've noticed a lot of packs rate the highest difficulty as "Extreme".