Go to the RiDE magazine's website. They may have a compete set of routes for you to download.
Have a look at lots of other threads on the subject on UKGSer. You ain't the first and you'll probably not be the last.
Calais direct to Flers is, according to Google, 410 kms (254 miles).
You have decided to tell us lots about why you are going, where you are staying, that you don't speak French, have never ridden there, that you're going to be on your own and departing on Tuesday, which - given that today is Monday and you booked last minute - may well be, tomorrow. Sadly, you have decided not to tell us:
1. What time you plan to roll off the ferry in Calais
2. What time you want to / have to arrive in Flers, though I guess it's the same day
The answer to those two secretive issues might possibly decide your all or part of your route for you.
3. Whether you think 250 miles is something that you usually knock off before breakfast or whether you think it's a fecking long way if you ride all the goat tracks and stop to gawp at every WW2 site along the way.
As time is possibly very short, here's what to do:
a. Fire up Google maps and ask it for Calais to Flers. That gives you the direct route, its direction, distance and direction of travel. If that alone is good enough for you, job done.
b. If not, fiddle with the Google map preference settings. Any good? Great.
c. Do the same in the free Via Michelin app, repeating as necessary, changing the preference settings. Happy? Fantastic **.
d. Then try something like https://kurviger.de/ and play about; you can't break it ***
..... And buy a map......
Over to you.....
PS If it's too late (and nobody else it seems has given you a specific route or routes) have fun. You'll be fine, I promise.