calculation for - 4 weekly salary for spouce visa

[q4question]

Hi all,

(1) i receive 4 weekly salary.
if i provide 6 payslips then those payslips cover approximately 5.5 months.
And if i provide 7 payslips then those payslips cover approximately 6.5 months.
how can i calculate exact gross salary for 6 months (or for 26 weeks) ?

(2) i got some payqueries and those were solved in later payslips by adding those missing hours (in next month's total hours).
Because of those payqueries, my lowest basic gross salary (within 6 months) was £1200 only. it means £1200*12=£14,400. How can i convince homeoffice ?
(Note: I work on zero hour contract. So, company can not issue a letter for certain yearly basic salary)

(3) i have done some overtime. but on payslips, it says "basic2". Should it be mentioned clearly as "overtime" or "extra hours" ?

if you can help me to find out the right answers, i will be thankful to you.