1-I always allow for expansion. Then I'm really nice and professional,so I AM the one who comes back.:)

2- Stick with whats there,if available.
I line up the top.

Except for the last one I did.
Matched the top of the tubs,in a basement.Installed the covers,original had a surface cover,mine was flush.
Man!!!! That 1/2 inch difference made the job look like crap! (in my eye.)