I have a flow that creates an event in a shared calendar for a holiday request based on having this MS form approved:

I need to add an action to update the circled sharepoint list column below (Holiday Days Taken), totalling the answers to number 4 above (i.e. total working days off), which I would do by an 'update item' action.

Question 1
How do I make sure every time someone submits a form, the column is added to not overridden?
For example -
User submits form & requests 3 days off. Update item action populates 'Holiday Days Taken' column as 3.
A month later, the same user submits form again and requests 7 days off. How can I get the formula to continually add the number instead of populating the 'Holiday Days Taken' column as 7. It should show as 10 (3 + 7)
Question 2
How do write the flow so that the system knows what person to allocate this to on the SP list? Presumably its a condition that matches the email address of the person submitting the form with the 'employee email' column. How do I do this?