It would be based on the following...
* The manufacturer making full schematics and service manuals available on the day a product is launched.
* Making all parts and specialised servicing tools available to anyone who wants to buy them.
* Not using unusual fixtures, fastenings or glues.
* Allowing people to downgrade any software if the feel a newer version slows down their otherwise perfectly serviceable device.
The more repairable a product is the less this tax would be, potentially dropping to zero. Crucially, the actual price of the products would play no part in the calculation. Cheap disposable Android tablets would attract the same rate that a high end smartphone would if they were equally as repairable.
I'd also levy a reduced, or even zero, rate of VAT on repair services to encourage repair over replacement.
I don't see a problem with letting people buy non-repairable devices, so long as they pay a levy for the waste it will produce.