Dies to Removal

So the breakdown would be closer to 2-3 and then 3-4?

I was trying to get at that same point by discussing mana counts by deck type. That's probably not specific enough since we're not necessarily discussing just lands. Creatures like Arbor Elf and Avacyn's Pilgrim or spells like Farseek and even Heartless Summoning count as mana sources/acceleration for the decks that run them.

I think every deck - regardless of type - wants to see a certain % of their mana sources in their opening 7 - that's why they play that many lands.

Can you think of any specific deck that's not true for? Like, do you know of a single deck that plays X mana sources and doesn't want roughly 11% of those in their opening hand?

No, not really. I was trying to write out an explanation that would make sense to more mathematically-challenged individuals like me.

Or we can simplify that further because deck types (aggro, midrange, control, combo, etc.) usually determine how many mana sources they want to run.

Decks with 24 or less mana sources typically want 2-3 mana sources in their opening hands. They can then cast the majority of their spells and creatures and draw into the sources that they need. These decks usually focus their gameplans on the first 4-6 turns.

Decks with more than 24 mana sources typically want 3-5 mana sources in their opening hands. They can then be assured of hitting their land drops to either let them cast their bigger creatures on or before curve or to give them maximum flexibility in casting their spells later on. These decks focus their gameplans on turns 4+.

Yes, no, maybe?

I base most of my mulligan decisions on land. Ideally, I want to see at least 2 lands, 2-3 early creatures, and at least 1 removal spell for my [mana]rb[/mana] deck.

For me, one-land hands are auto-mulligans. The risk of not drawing enough lands to remain in the game is too great at that point. Similarly, a hand of 5 or more lands is an auto-mulligan. I will sometimes keep 4-land hands because then I know I can cast the majority of the creatures in my deck with an excellent chance of drawing that 5th land when I need it. I also have a decent chance of drawing creatures or spells in the first few turns.

Complicating factors include the types of lands in that opening hand. If I open a hand full of Dragonskull Summits or Godless Shrines, then I'll mulligan since that throws me off curve. If I see one red and one black
source, then I'll usually keep that hand depending on the creatures. If I see a Blood Crypt and a Dragonskull Summit, then I'll keep since I can cast any combination of early creatures and spells I happen to have.