Yes. A piece of the same stock will do. There may be an error as a result of the original stock not being an exact thickness over the entire surface, but it should be small.
This suggestion is on the basis that you have already begun the job and started the roughing cut.
If not, set Z zero to the bed before you create the toolpaths. Any variation in the stock thickness is dealt with on the roughing cut, but this is still not perfect.
VERY best practice is to mill the stock flat before the roughing cut, that is for perfectionists only!