IF formula help - to add dates to each other to get longest difference?

Hi guys, I am no excel expert, only to A Level standard and am really struggling to figure out a formula or macro. I need to work out the length of notice to give to employees that are being dismissed by comparing 2 sets of criteria and working out which would be more beneficial to the employee, if that makes sense? (I am assuming the formula will be really complicated!)

I need to work out what the employees last working day will be.
Say we dismiss on 23rd May
+ 2 days (for postage of letters)
+ extra notice. This is worked out comparing the employees length of continuous service versus grade/salary. The formula needs to compare the two and work out which is most beneficial to the employeee.
1 years employment = 1 weeks notice
12 years employment = 12 weeks notice (no matter what grade)
grade 6 and below = 4 weeks notice
grade 7-11 = 8 weeks notice
grade 12+ = 12 weeks notice
The formula needs to work which would give the employee a longer notice period then add the notice required to the dismissal date (23rd may) and calculate the employees last working day.

For example if i had to dismiss an employee on 23rd May and they had worked for 2 years on grade 6 they would get 2 days plus 4 working weeks, so their last day would be 22 June.
But if they had worked for 12 years on grade 6 they would get 12 weeks notice plus the 2 extra days.

Please see the attached spreadsheet for an anonymised sample.
Any help would be really appreciated as I can't seem to get the formula to work!

Thank you! :-D