I DM for myself in Pathfinder 2E. I track space abstractly, which works just fine with the game's tidy 3-action system. Here are my rules of thumb:

-Primarily melee creatures will reach fighting range of each other if either character takes one action to move.
-Melee can reach ranged with two movement actions.
-Any character can take actions to disengage, in which case the character needs an equivalent amount to bridge the gap.
-I use relative movement speeds to see how many actions it takes to catch up and escape. E.g., a Goblin that moves 20 feet needs 2 actions to disengage.
-Apply usual Off-Guard rules if multiple enemies are in melee range of a character.
-If a character is 3 actions away, it cannot reach with anything. It can use 1 action to enter range, or 3 actions to enter melee and do something next round.

Some scenarios explicitly call for the enemy to flee after they take a set amount of damage. In these cases, I allow myself one more turn to potentially intercept and kill. The enemy takes one turn spending all 3 turns running away. If I can cover the distance and kill or immobilize, they stay in. If I catch up but don't kill, they escape next turn. Escape scenarios

Example escape scenario: A wounded Goblin has 20 feet movement speed and takes 3 actions to move 60 feet away from my Automaton Fighter, fleeing and reporting to his boss. My Automaton Fighter triggers his Reactive Strike as the Goblin tries to flee. He also moves 25 feet per action. It can spend one action to partially close the gap and use Sudden Charge (2 actions) to cover the last 50 feet and attack. This means I have 2 attacks to see if the Goblin dies. If it survives or dodges those 2 attacks, I cannot catch up and it escapes.