Government schools have clearly failed to educate children. Then it is up to those people to spend or not spend that money as they want, whether it is to provide private schooling, home schooling, or buy a set of encyclopedias. If the government is in charge of distributing the funds to private schools, then the government will insist on dictating the policies of those schools. That will only result in private schools becoming as bad as public schools. Even if the theoretical savings were possible, the advertised application of that savings to tax reduction is pure fantasy.
Subsidies always increase prices and the one who pays the piper always calls the tune.
This formula has not yet been implemented to analyze the actual spending for all 50 states. This is an extremely large unknown that needs to be immediately quantified. It guarantees minimum funding for an extremely bureaucratic public system of waste, fraud, abuse, and low-performance. No true conservative would advocate gov't doing such to keep poorly managed private sector companies operating and, thus, they shouldn't advocate gov't doing such for anything in the public sector. The guaranteed minimum funding ensures the public education system, which delivers a marketable commodity, never goes away when it should be totally turned over to the private sector.