Odoo Payroll and Leaves Customizations

I need to do some customizations in "Leaves" & "Payroll" modules. I need the system to automatically allocate a specific number of days as leave for each employee, for every day he worked and I need to do some formulas in the payroll calculations I need Odoo to follow the UAE labour laws 2 days per month, if they have completed six months of service but not one year 30 days, if they have completed one year of service. This is only for new employees that have joined from six month to a year. For other employees that have been in the company for more than 1 year, for every day of the week they worked and weekends (with no work), they generate 0.083 days holiday also, the leave salary should be calculated automatically as a 1 month salary (basic only), and added to his salary sip, only after 11 months of service, after that employee has returned from his previous leave also, sometimes an employee will apply for 30 days leave, but he will come back after 35 or 40 days, I need a way for the HR to enter the Return From Leave date and to be approved by HR, Department Manager and Director. And the extra days took should show as negative in his remaining leaves Also for the monthly payroll, I need a formula to calculate the total salary (Basic + Allowances) according to the days worked per month, for example if I want to generate a salary for only 15 days, the formula must check how many total days are in this month and calculate the salary accordingly, some months are 30 days, some are 31. but if the worker works the complete month, if it's 30 or 31 days or sometimes february 28 days, his total salary will be released.