Adjust it week on week, a mistake people make with budgets is assuming that its fixed costs over the year. The way I work it, is I have a minimum and maximum weekly spend. The maximum is based on heavy spend weeks month end when bills need paying.
If your a contractor consultant with a gappy work year eg six month contract, month gap, three month contract, two month gap etc... project forwards a minimum weekly income, based on a 53 week year (a year can be more or less than 52 weeks depending on jan 1st, dec 31st and leap days). If your minimum weekly income is greater than your maximum weekly expenditure, good. Otherwise push forward the difference between minimum weekly spend into the following week and so on until you have enough for the maximum outgoing
Mondays Wednesdays and Fridays, check your bank statements, use that to adjust your week in/out accordingly.
Finally, switch to an umbrella company and let them manage taxation VAT and expenses, its generally a month on month cost rather than the big bill from the accountant at year end if you're a limited Co. I have way to many contractor/consultant mates who have a limited Co just so they can say they are a director on linkedin (a vanity thing).
Managing a budget is a hard task, that why companies have bean counters to sole watch the business budget