Actually, 30 to 10 would result in your having two more turns, i.e. having 3 turns for the enemy's one.
This is the rule of thumb that I use. I know _nothing_ about the code, it's just a theory based on observations, which I have not disproved yet.
Think of your units as having an energy tank. Lets say that it takes 100 energy units to fill it. Every 'tick', every unit generates energy units equal to its initiative. When an unit has filled its tank, it moves.
So, you have two units, A with initiative 36, and B with initiative 13.
On the 3rd tick, A moves. It moves again on the 6th tick. On the 8th tick B moves. On the 9th, it's A's turn again. And so on.
How accurate is this mechanism? Very. It perfectly predicted the fight I just tested up to the end.
Does it break down at some point? Yes. In large fights, when Haste/Slow comes into play, when units change sides, when units use some special attacks.
But in general, it works really well for designing a strategy, and for estimating the frequency of acting.
This, and a lot of what I know about the game can be found here. I just wish I knew close my theory is to the truth.