I would be grateful for help in construing this famous quotation. I take sat to be an indeclinable adjective, but it can also be an adverb. Here it modifies celeriter “quickly enough.” I think that fieri is the infinitive passive of fio which, in turn, comes from facio “to do.” Similarly, I take fiat to be the 3rd sing. present subjuntive of fio. So would it have a jussive sense “let it be done”? I believe I understand satis bene “well enough.” Where does quidquid go? Is it the subject of fiat? Or does it go with the infinitive fieri “whatever is to be done quickly enough.”
It’s the subject of fiat, so 'What ever is sufficiently done (or done well enough) quickly is done well"
I can imagine this being said by a consul whose legion is under attack, and manages to hastily put up the standard fortified camp, but not with the ‘standard’ measurements and neatness. The camp, however is good enough to protect the legion (i.e. does it’s job adequately), so it is done well.
Thank you very much.
I like a different translation, one that sees the quotation as a warning against reckless haste: “What is done well enough will happen fast enough.” OWTTE
Hugh is right.
"Nihil autem minus perfecto duci quam festinationem temeritatemque convenire arbitrabatur. Crebro itaque illa iactabat:
σπεῦδε βραδέως· ἀσϕαλὴς γáρ ἐστ᾽ ἀμείνων ἢ θρασὺς στρατηλάτης
et: “sat celeriter fieri quidquid fiat satis bene.” Proelium quidem aut bellum suscipiendum omnino negabat, nisi cum maior emolumenti spes quam damni metus ostenderetur.
He thought nothing less becoming in a well-trained leader than haste and rashness, and, accordingly, favourite sayings of his were: “Make haste slowly”; “Better a safe commander than a bold”; and “That is done quickly enough which is done well enough.” He used to say that a war or a battle should not be begun under any circumstances, unless the hope of gain was clearly greater than the fear of loss; "
suetonius-lives_caesars_book_ii_deified_augustus Loeb 1913.
The context of all these statements is caution against being too hasty when the outcome is uncertain.
One thinks too of Macbeth “If it were done when ’tis done, then ’twere well
It were done quickly”.
One might say my haste got the best of me. 
I realize now that I put one bene too many when I translated. I thought the phrase looked familiar, but couldn’t remember from where.