Argthtjtr

Grammarist & Our beloved StackExchange both say that the phrase "Chalk it up to" dates back to, among other things, debts being tallied on a chalkboard. However, when I hear the phrase "chock it up to", I get a feeling that something is being supported or something is supporting another idea, i.e. a chock.

For example, I made a web form at work that people consistently complete incorrectly. I feel the cause is because I wasn't explicit enough with the directions. Therefore, I "chock the incorrect completion up to" my mistake given my support that I wasn't clear on the directions.

Which is correct?

Robert Rubin, Going to Hell in a Hen Basket: An Illustrated Dictionary of Modern Malapropisms (2015) has this discussion of "chalk-full" and "chock it up to":

chalk-full V: chock it up to. Confuses chockfull with chalk it up to. Chock-full is an old phrase, perhaps coming from choke-full or full to choking. Chalk it up to comes from chalk tally marks on a slate. Chock marks indicate where to put wooden chocks (or wedges) and may be confused with chalk marks.

A Google Books search finds more than 40 unique matches for "chock it up to" in which the author presumably meant to say "chalk it up to" but either spelled chalk wrong or didn't know the correct traditional spelling of the idiom. Tellingly, the vast majority of these 40+ instances come from the past ten years, suggesting either that chock is emerging as a legitimate variant spelling of chalk in this situation or that many publishers no longer employ copy editors to find and correct mistakes of this type.

To complicate matters, a Google Books search finds a number of instances of "chock it up" in which chock means "to stabilize with another object in order to prevent [a thing] from moving. Thus, for example, from John Weiss, Life and Correspondence of Theod. Parker, Minister of the 28th Congregational Society, Boston (1863):

This writing-desk I sit at {at Montreux} is so made that a book will slide down the leaf, and I must chock it up.

From "Mending the Road From Rich Mountain," in The Breeder's Gazette (March 26, 1913):

"Well, wait, I'll call my man out of the field," said King Wheeler.

"No do not do that; you and I can manage it [removing an obstructing stone from the roadway] very nicely. Please get down and chock it up with this small stone as I pry it up and presently we will be able to manage it."

From "Flight Quarters": The War Story of the U.S.S. Belleau Wood (1946) [combined snippets]:

At the foot of the elevator a dollyman waits. To the commands of: "To the starboard", "To the port", and "Hold that, you wanna break a wing?" the hangar dock crew pushes the dud in spot. "Chock it up", the coxswain orders, and while the two mechanics roll up a stand underneath the engine a trouble shooter jumps into the cockpit.

And from Industrial Supervisor, volumes 22–24 (1954[?]) [combined snippets]:

Secondly, when you pick up small heavy boxes, like small crates of machine parts, you run a chance of getting your fingers smashed. So before you pick up such an object, chock it up. Stick a chip or a piece of wood under it to raise it from the floor a little bit so you can get your hands under it for a good grip.

Conclusion

The idiom "chock it up" can be correct in instances where it means to insert wedge-shaped blocks or other objects next to something to prevent it from rolling, dropping, or coming loose; but it is not correct—at least not yet—in place of chalk it up in the idiomatic expression "chalk it up to X."

Chock it up (or chuck it up to) is an eggcorn. Given your example sentence, it should be chalk it up:

To credit or ascribe:
Chalk that up to experience.

(AHD)

Some examples that prove anglophone journalists make mistakes just like everybody else:

“Chock it up to just another amateur exhibition of a lack of administrative ability,” said Georgia pollster Claibourne Darden. (John King, Associated Press, The Daily Gazette, Schenectady, NY, June 4, 1993)

Chock it up to the wildly popular Visa check card, which accounts for about one-third of all Visa dollar growth volume. (San Francisco Business Times, Mar. 26, 2003)

(The Eggcorn Database)

For example, I made a web form at work that people consistently complete incorrectly. I feel the cause is because I wasn't explicit enough with the directions.

In this case you can chalk up (not chock up) the constant problems people have with your form to unclear instructions.

It's chalk up.

To chock something is to put wedges against its wheels to stop it moving. However, this doesn't support your interpretation of "chock it up to X" as meaning "support it by X" because the verb "to chock" doesn't take an indirect object. You don't chock something to something else; you just chock it. The only way it makes sense to say "chock it up to X" is when "to" is used in the sense of "for the purpose of", e.g., "chock it up to prevent it moving". But that's clearly not the sense intended. And, in all of these senses, the word "up" is redundant: "chock it to prevent it moving" carries exactly the same meaning.

You might want to think about comparable words. Does "tally it up" or "hold in place" work better? In your example, both could technically work if you're using "chock" as a replacement for "immovable idea/dogmatic belief" but "chalk" works better because you're outlining the steps of logic you took and tallying - or "chalking" - them up in order to arrive at your conclusion.