Upvote:2
As you will support your application with bank statements, you may want to show your regular income as the fixed salary from your full time position.
Where you are asked for other source(s) of income, provide an average amount per month derived from your part-time position.
From the accompanying bank statements, an ECO can assess funds available to you. You wouldn't want to inflate your income by what you might hope to get but, rather, you're giving a view of your actual earnings over a period of time.