I'm a Thai and just done helping my husband filing his Non-O (Marriage) Visa for the first time. I hope the information I got from my local IO might help. BTW, please excuse my English.
If you are applying with the 400k option, on the official document 'checklist', it is stated that you are required to show that you have no less than 400k deposited for 'the whole 3 months until the date of the application', and a letter from the bank to guarantee that you 'currently' have no less than 400k deposited in your account.
The IO told me that the amount of money in the account must not go below 400k anytime during those 3-month period.
However, there's a remark says 'only for the first-time applicant, the applicant must show that he has 400k deposited in his account for over 2 months.'
Though the rules seem to be applied differently from one immigration office to another, I recommend your wife to contact your local office early in person to check. It will save you so much time.