Interesting idea. Thanks for sharing it.
As you may have noticed, the count easily becomes inaccurate because you assume that when a column is calculated, it applies to all the items in the current tab, which is not always the case (for example, enter a folder, enable the column (the count should be OK); then expand a subfolder: the count will not be OK, since some of the items will not be calculated again).
I don't think there's an easy way to know in advance how many items are actually involved in the calculation, as discussed here:
Or maybe do you have an idea to workaround this?