Hello,
I suppose, that biters levelling could be a little more flat.
After setting up Beginning enemy level 1, Ending enemy level 7 and Variations as 1 I got the following chances at evolution 82%:
https://imgur.com/a/FFacks1

Maximum tier biters started coming at evolution 80%. I expected them after 95% .
To simplify things let's assume that 6 level increases are evenly divided by 100% of evolution. Then new upper tier should appear every 16.67%.
So max tier should start appearing rarely at evolution 83.33 and become 79% of all generated enemies much later - somewhere about 95%.
Am I wrong?

Neither of those should be max tier units.

https://github.com/veden/Rampant/blob/master/prototypes/SwarmUtils.lua#L289
Is the probability table that I use.

I generate unit tiers with the following function
https://github.com/veden/Rampant/blob/master/libs/Constants.lua#L449

when I use level 1 and 7 as the values for tierStart and tierEnd I get the following sequence.

1.0
2.0
2.0
3.0
4.0
4.0
5.0
6.0
6.0
7.0

so at 80% evo you should be seeing the level 5 units
so at 85% evo you should be seeing the level 6 units
so at 90% evo you should be seeing the level 6 units
so at 93% evo you should be seeing the level 7 units

The biggest size units are called "Juggernaut".

Let me know if this answers your question.

I'm sorry. I thought, that Juggernaut will be a class name for biter with Tier10 attributes. Class names misled me to wrong conclusion. Now I see, that 10 classes are always generated with all their names, but under their masks they have attributes of calculated level according to Start/End Tier selected. So my neutral Juggernaut biter will have not 30000 health, but 3500.
Yes, you are right, at 82% evo biters have health of 1200-1800 - that goes correctly to level5. And at 85% attackers will become even stronger... Combo with Deathworld and fast evo settings makes it so fatal, will try with Default evolution settings.
Thank you a lot!

in the next update all spitters damage is getting nerfed by roughly 75% and biters damage is getting buffed by 70%.

Currently spitters do to much damage and walls are like paper.

Also adding a warmup to spitters, so they don't shoot the tick they get in range.

Spitters are far stronger then biters in the current build.

Yep, spitters are doing the hell from behind positions while biters and snappers are slowly moving close to walls taking damage from all turrets and reflecting with their defenses. Factorio turrets have no capabilities of selecting appropriate enemies in accordance with their damage type. In vanilla it was a balance - stupid enemies vs stupid turrets. In Rampant enemies are smart.

in the next update all spitters damage is getting nerfed by roughly 75% and biters damage is getting buffed by 70%.

Currently spitters do to much damage and walls are like paper.

Also adding a warmup to spitters, so they don't shoot the tick they get in range.

Spitters are far stronger then biters in the current build.

Yes, now spitters are much less dangerous - I can stand still under fire from dozen of lvl5 spitters (neutral/acid) with armor MK2 as it decreases more damage points than they do. I hope the same will be with lvl8 spitters vs MK3 armour. :-)
The most sensitive things are spitters' warmup (you really do a great balancing by slowing them) and range.

I don't get the range calculation for their levels:
Lvl4 has range 14, while lvl5 spitters have range 11 (checked for neutral and inferno factions). In SwarmUtils.lua I see range values of 14 and 15. The opposite happens for worms: lvl4 has range 37, while in SwarmUtils.lua it is written as 33. If worms lvl8 have range of over 41, than I'm in trouble...

In other words if spitters eventually get range of over 20 or worms get the range of over 41 by max level, then even small near wall nesting team of mobs will make a disaster vs endgame defenses thus making gun turrets mk2 and advanced lasers completely useless and player has to replace them with rocket + cannon turrets after evo 90%.

The numbers in swarmUtils represents the base attribute.

An entity can have between 0.7 and 1.3 with a standard deviation of 0.17 of the base attribute,

a range of 34 base attribute, could be between 23.8 and 44.2 with standard deviation of 5.78.

Open to feedback or changes.

At lower levels, a 30% deviation is almost necessary to see any variation at all. At higher levels, a 30% deviation is almost massive. Might need to shrink the deviation as the tiers increase. Something like (70+Tier)% to (130-Tier)%