The skill Hide says that a creature "can move up to one-half [its] normal speed and hide at no penalty[, but w]hen moving at a speed greater than one-half but less than [its] normal speed, [it'll] take a −5 penalty" on Hide skill checks made to hide (Player's Handbook 76).
What I'm struggling with is this: In what fashion is the creature's speed halved? That is, what if the creature only needs to hide in a few—rather than all—of the squares it enters? Does the creature declare at the start of its movement the speed at which it will be traveling? (Note that the game usually mentions when something mandates a declaration of intent.) Alternatively, do just those particular squares count double? If this latter, since movement is dynamic rather than plotted beforehand, what happens when the scout later in that move action opts to move his full speed (rather than 5 ft. less) after having hidden in those particular squares? (That is, the creature seems to avoid any penalties completely if the squares are near enough to his starting position!) Or is there some other solution I'm overlooking?
The speedy scout wants to use the supernatural ability hide in plain sight (yes, that one—don't judge) combined with the feat Spring Attack (PH 100–1), both to increase the accuracy of his attacks (a foe is treated as flat-footed against a hidden attacker and the scout'll have a +2 bonus on his attack rolls as a hidden attacker is effectively invisible (Rules Compendium 92)) and to avoid retaliatory strikes from the foes he recently attacked (even if a foe determines his location, the scout's total concealment'll give the foe a 50% miss chance).
The scout's current plan is to approach a foe while not hiding, employ the special ability hide in plain sight to hide in a square 10 ft. away from his foe, use the rules for Sneak up from Hiding (ibid.) to move adjacent to his foe, and attack his flat-footed-with-regard-to-him foe. Then, afterward, because of the Spring Attack feat, he'll visibly retreat from the foe and, using again the special ability hide in plain sight, hide in a square some distance away. Thus the scout only really needs to hide in two squares during his turn. The scout's normal speed is 50 ft. How far can he move during his turn using this plan of attack without suffering any penalty to Hide skill checks? While suffering only a −5 penalty to his Hide skill checks? And—darn you, Hide skill!—what's his penalty if he moves his normal speed?
Note: This linked question is about how the feat Spring Attack and the special ability hide in plain sight work in conjunction. Because of that linked question, I know how those two items work in conjunction. What that questions does not answer—and what this question asks—is How can I determine the creature's speed when the creature need only use the Hide skill in few of its squares? That linked question doesn't address this speed issue at all.