As you know autumn Mid September to November and spring (March to May) are the best time for the Everest trip. Normally, the weather is pleasant with a crystal-clear sky and amazing Mountain View at this time. If case weather is not favorable to fly back to Kathmandu on your schedule then those two days will cover getting back in Kathmandu for your further destination. We recommend you put at least one more day but you already have two spares. So, two days is good enough.