How many days of H1B should be leftover for extension?

[bukli]

My future employer has filed PERM in January 1^st week 2018. My H1B with current employer is maxing out in March 31. Upon recalculating the vacation period, I will get another 40 days.

I am sure I have to leave the country.

But in order to apply for extension after I-140 approval, How many days of H1B should be leftover ? When should I leave the country to avoid any issues ?

Please advise. 

-Bukli