Hi David,
What I meant by my question was which category under the Financial Requirement are you thinking of applying under.

Your specific employment conditions make a significant difference to the category and the documentation needed to be submitted.

In principle UKVI do allow overtime etc
As always there's a number of "if's and but's" for you to validate. Plenty of different employment cases around.

Anyway here's their stated starting point general requirements:-

Overtime, commission-based pay and bonuses (which can include tips and gratuities paid via a tronc scheme registered with HMRC) will be counted as income from employment where they have been received in the 6 or 12 months prior to the date of application as applicable. Sometimes the person will receive the same amount of income from overtime each month; sometimes overtime payments will vary, with different amounts (if any) each month. All overtime in salaried employment will be calculated based on the approach to income from non-salaried employment. This will be an annualised 6-month average for the overtime which will be added to the level of the gross annual salary.
As there's some detail you must validate to be sure, I'd strongly suggest reviewing the Financial Requirement to see exactly how it sits with your own employment circumstances.

Also you need to be 100% sure of how your employer will present their supporting letter.
We've seen a number of members here let down by the employer statements.

Hope it gives you a good starting point