If it was me I would cut the batting 90 x 100. You can't go wider
than 90" anyway. So if it shrinks a lot and you are short one way
you can always add another washed piece for length.
Are you planning to wash in cold or hot water?
I wash my quilts in cold water and get about 3% shrinkage with W&N.
Might be a bit more in hot water.
Please let us know how it comes out. I haven't done my experiment
yet. But mine will be with Warm and White which shrinks more than
W&N.