It's more involved than a simple countdown, however. When a unit hits 0, that unit gets a turn and every other unit gets their count reset to 1000 and whatever remaining counts they had are added to that 1000.
So a unit that didn't hit 0, but was the second lowest wouldn't necessarily be the next unit to move. In most cases they would get the next turn, but due to the reserve system, they can be passed by other units as the turn order is very dynamic.
Turn Order Factors
The more Speed you have, the faster the count will tick down and more turns your unit will have. This has multiple factors to ensure a balance.
Each turn a unit will regenerate a portion of MP which is used to cast abilities. The longer the time in between turns, the more MP the unit will receive when their turn comes, so being slow and steady can come in handy.
Base Count, Speed, Weight, Penalties
The number 1000 that is used is pretty arbitrary, it could just as well be 100 however due to Speed value parameters and things like item weight, 1000 works pretty well.
So we know that Speed determines turn order, but it's not the only thing. Any equipment currently equipped on a unit will have a Weight value. This is a value that depicts how much this item slows you down. With equipment, the bigger it is, usually the more defense it grants or attack power it has, so this serves to balance weapons and armor. The amount it slows you down is minor, but you will notice higher Speed units not getting as many turns, so it's worth keeping an eye on.
The second, and larger factor is what your unit does with it's turn. Consider a unit's turn comes up and it simply decides to Wait and end it's turn. This unit will incur NO extra penalty for movement, or action as it did neither. This means it will be reset to a count of 500 instead of 1000. Essentially it's next turn will come much sooner.
As of the writing of this topic, the extra penalties for movement and acting are set to static values, meaning it doesn't matter how far you move or what action you take, you will always get the same penalty added to the base 500 count.
For movement, the penalty is currently 200. Actions such as using an item, attacking, or using an ability adds 300. So for example, if you attack without moving and then end your turn, your unit's count will start at 800. If you move and act, the counter is reset to the full 1000.
TODO: Sliding Penalties
What I will eventually incorporate is a more dynamic penalty. Rather than a flat value, the penalties will be based on the amount you move, including vertical movement, and for actions, each one will have a specific value. The latter adds a neat way to create abilities that are very powerful, but have major drawbacks.
Managing who moves when and what actions they take will have long term effects on the outcome of battle.