More numbers!
View attachment 402
So, E(land) t4 is the number of lands you expect to have drawn by turn 4, provided you're on the play, play 17 lands, and no other card draws. Add to that the number of lands in your opener, and you know how many lands you have. The 1,27 number for Magmatic Insight includes the land you have to discard as part of the cost. You're really only a fraction behind the normal expected progression, less than 0.1 land! Say you start with 3 lands, then on average you will have 4.27 lands on t4 if you cast Magmatic Insight before t4, or 4.36 if you don't. In short, it's entirely defensible to use Magmatic Insight even at low starting land counts in your hand. I would probably not cast it when I start with only two lands, there is a 15% chance you won't have a second land drop on t2, and that's too big a risk imho, but if I start with 3 I would definitely cycle a land away. The odds off that backfiring are a lot lower than the odds that it draws you into a key card or past a land glut. Good card is good!