adb wrote:

Or even better yet...weigh a shelf full's worth of books. (ie, stack
textbooks until they're 48" high on the bathroom scale) That'll give
you an accurate weight to plug into the sagulator.
(http://www.woodbin.com/calcs/s agulator.htm )

But, as was discussed in a thread here not long ago, the "sagulator" is
based on homogenous material and (to my surprise) the only plies that
add significantly in ply are those w/ longitudinal grain--the others
only add roughly 20% or so. So, one would have to look at the plywood
construction in order to estimate the proper dimensions to plug in.
Those would be only roughly 1/2 of the actual thickness so that the
two-1/2" sheets would be equivalent to only roughly 3/4", maybe...

I posted a link to the US Forestry Research site w/ a detailed paper --
a Google over the last week or so should find it.