On a 5-foot grid, you can move two squares diagonally for 15 feet of movement, i.e. sqrt(2) = 1.5.
When there’s an ambush, the ambushee with the highest initiative repeatedly rolls initiative against a DC based on how good the ambush was, while the ambushers just take turns in whatever order they want. When the ambushee beats the roll, we set up a normal initiative order. Also, initiative is decoupled from dexterity – being good at turning cartwheels or picking pockets doesn’t mean you won’t freeze up in combat. Initiative is its own stat.