There is no way to calculate performance during cruise with one or more gear extended unless for some reason your AFM provides it.
You simply have to start cruising and see what the plane will do....
Chances are you can't land anyway because you are overweight so you have some time to figure it out.
If you are heading out over land with plenty of viable alternates along the way this isn't so much of a problem. If you are headed out over open water or hostile land forget about it.
If there is a good maintenance base on the way you can use as a contingency before having to make the final decision that's even better.