First some background on the tax itself . . .
In 1782 the General Assembly of Virginia enacted a major revision of the commonwealth’s tax laws. The act provided for statewide enumeration on the county level of certain personal property and land. It also created a permanent source of revenue for the operation of government in Virginia.
. . .
Various revenue acts passed by the General Assembly established the tax rates and procedures for the collection of personal property taxes. At first, justices of the local court were designated to collect the taxes. By 1786, the county courts were directed to divide each locality into precincts and to appoint commissioners to collect the taxes. Taxes were assessed between March and April of each year and were payable by the end of December.
The commissioners prepared four “fair and correct” copies of the personal property tax books. Copies were prepared for the commissioner, county clerk, sheriff, and auditor of public accounts.-Using Personal Property Tax Records in the Archives at the Library of Virginia
by Minor T. Weisiger
Library of Virginia 2010
So with that in mind . . .
1- It appears then that the losses of slaves occurred between Spring 1863 and December of that year.
2- The total number of escaped slaves mentioned is 213. Valued at some $134,472 Confederate dollars.
3- Only slaves over twelve years of age were usually counted on the Property Tax books so I assume only slaves over twelve were counted here.
4- The 1860 Property Tax book,the closest to 1864 of which I have full scans, counts some 1808 slaves. The slaves referenced here would then make some 12% of that total.