Best Movie Setting

[Tuor of Gondolin]

Both above are good points on Theoden's charge. As to Eomer's
movie charge I agree. I assume the rather absurd steep angle was
PJ's penchant for dramatic exaggeration, but you could also argue that
the orcs cracked psychologically, as could well happen at the end of
protracted combat when unexpected new enemies appear.

About the extent and effect of Theoden's charge, this occured to me
before the movies or the publishing of the Atlas of Middle-earth. Of
course Tolkien meant for it to be a desperate charge, but one reason it
was desperate was the defenders nonknowledge of imminent relief.
In this sense you can postulate a psychological effect on Saruman's army akin to the movie Eomer's charge, that is, when the book King sweeps the orcs and crams them past the Great Dike. Certainly Saruman's army still had numbers, but you can at least suggest the possibility of their panicking and, without the ents and huorns blocking them, retreating back across the Isen. History is replete with examples of superior numbers pschologically cracking:

The hosts of Isengard roared, swaying this way and that, turning from fear to fear. Again the horn sounded from the tower. Down through the breach of the Dike charged the king's company. Down from the hills leaped Erkenbrand, lord of Westfold. Down leaped Shadowfax, like a deer that leaps surefooted in the mountains. The White Rider was upon them, and the terror of his coming filled the enemy with madness. The wild men fell on their faces before him. The Orcs reeled and screamed and cast aside both sword and spear. Like a black smoke driven by a mounting wind they fled.

So without the huorns present it seems at least tenable that the orcs/wildmen might have retreated. The real problem, I'd suggest, is,
even if Theoden's forces managed a costly and hard-fought tactical victory, then
without the ents destroying Isengard Rohan would have found it necessary to
detach forces to guard it, hence leading to a later and weaker relief force to
MT, with obviously icky consequences for the good guys.

Hey, just a little "counterfactual" history surmising.

Oh, and as for the numbers postulated in "The Atlas of Middle-earth", the
raw numbers of c. 1,800 survivors at Helm's Deep seem tenable, although you could argue for perhaps 1,400-1,600 effective fighters by sunrise, and you can
assume most of the horses were fairly protected before the charge, but getting
at least 500 + organized to charge quickly during a siege does seem a major
challenge, but then it is the Realm of the Horselords.