I was rounding to the nearest .25 doing it in my head , but if you want to get truly technical: 20->10->5 ->(1.66 saved)3.34->(.53 parry)2.81 dead The other advantage to running deep at first is that you can combat reform to expand your frontage (more models attacking), but you can't combat reform to reduce your frontage to increase ranks. Theory crafting in a vacuum aside, you will see better results using a combined arms lizardman list. Magic (like dwellers below), blowpipes, and salamanders to soften up big units of enemy infantry, deep units of saurus or skrox to hit them, break their steadfast, and then run them down.
All maths aside, you say you have actually tried a block of 30 with spears? How did it work out? Did you find it worth its points? Steadfast is indeed huge this edition, but remember if you are winning combat through kills steadfast doesn't matter. I'd run less ranks but 10 wide against a unit of say 40-50 Skaven, rather than more ranks but 5-6 wide. I know the saurus are going to dish out a huge amount of kills and will win combat. The Skaven will stick around from steadfast, but next round is no different, I will kill enough to win again until either support for one side arrives or I cut through them all.
My current set up is 29 saurus with spears, FC, with a scar vet lt armor/shield, sword of strife, dragon hem. Total cost for the unit is 521 points.In a 6x5 formation it dishes out 23 S4 ( 17 on the charge)attacks and 6 S5 attacks, with a static res of +4. I don't start losing attacks until I've taken 13 casualties. I've yet to have a combat last longer than 3 combat rounds, and between winning combat by 3-4 and breaking steadfast with my extra ranks (meaning they test at -3 or -4 even on their generals leadership.) I've managed to run those units down instead of getting tar-pitted for several turns. It's not quit as nasty as some of the deathstars from 7th ed, but I use a combined arms approach to building and playing my armies; Doing that really boosts your overall output and increases your chance of winning. Magic and shooting weakens up anything that might tarpit, then the saurus come in to finish it off while my ranged stuff goes on to the next target. I'll write out my current list and edit in the link in a few minutes. Edit: http://www.lustria-online.com/threads/current-2500-all-comers-list.4600/ Actually, it does matter. Steadfast means that no matter how much you win by they'll always take their break test on their base leadership, but with deeper ranks even if you win combat by less you'll start making them take tests on lower leadership... and if you break them and run them down they no longer have a chance to deal casualties back to you. It also means you can move onto the next enemy unit and do the same to them rather than waiting for support from one side or the other to show up to your first combat.
Yeah thats a good point actually, I didn't consider that they might break easier if you take a bunch more ranks. I guess in my first couple of 8th ed games I have become used to being completely outranked by my opponent's horde armies.