Hi,
This is the curved, uneven shell whose underside I’d like to fill and make flat. Is there a quick, easy, and clean way to do it? Perhaps there is a brush with a stroke height I can custom set and it would fill it to the exact height with the stroke of a brush?
Second question… Is there a way to increase the size of a brush past the 1000 draw size? For some reason, my brush size is at the highest 1000 draw size right now but it looks tiny in relation to my geometry.
thanks 
Brush size: is “dynamic” active? There’s a button above the brush size slider. You could also try to scale down your model. (in the deformation palette)
By underside do you mean the part of the model that is facing away from the camera in the image? Try the different “flatten” modifiers in the deformation palette
“Preferences > Draw > Max Brush Size”
thanks … those helped 
btw… the Flatten modifiers in the Deformations palette (Flatten with z option selected) seem to flatten by moving/removing geometry. Do you know of another flatten method which flattens by adding geometry in the hollow underside to fill it up? The way the ‘Flatten’ modifier seems to work is by pushing/moving geometry halfway into the geometry. If the geometry is considerably thin and uneven, it seems to flatten the object entirely by the time you get the underside flat. That affects the shells outer edges if you get what I mean.
You may find the TrinRect brush (hold Ctrl+Shift to select) will do what you want. See my post here for some info:
http://www.zbrushcentral.com/showthread.php?186611-zbrush-how-to-close-regular-surface&p=1087196&viewfull=1#post1087196
Also here:
http://docs.pixologic.com/user-guide/3d-modeling/hard-surface/clip-brushes/trim-curve/
thanks Marcus… that will really help ^^
Another method you could try is doing an extraction.
Turn off Double, and use a negative value (like -0.1).
Then you can flatten the extraction on the z-axis.
Of course, you’d have to combine/merge the extract with the front when your done.
