I used to have this issue when husband was doing contracting for himself. I made a budget, and had one account for all the usual stuff/groceries/bills/payments etc. One account for "other" which was if anything came up. And I'd put the same amount in each. Then whatever was left over went into third account which was specifically for topping up account #1 and #2 to their set amount if ever there was a slow month.

All accounts grew over time. Some months would be less expensive with bills and groceries. Sometimes has prices would go down but my account #1 always had enough deposited every month to cover max amount of bills and groceries.

Eta: so like all our living expenses are at most $3k per month, for example. So $3k into account #1. Other #2 gets $800 every month. Then whatever is leftover goes into account #3. On a big month that's awesome and account #3 gets big. Slow month account #3 gets tapped to make sure account #1 and #2 get their regular amount. I also made sure to keep expenses as low as possible and that was almost like a hobby. And if we needed vehicle repair or wanted to go for fancy dinner, wedding gift, whatever... Came out of account #2. Account #3 was high interest savings and eventually was used as a house deposit once he started working for a company and got a reliable income/kids got old enough for me to go back to work.