Quick question about having a companion voucher, booking 2 separate flights and then calling for an Avios rebate.

Is it best to:
1) Book outbound and return using full avios, before calling for a rebate
2) Book one of the legs using the voucher, and pay full avios for the other leg before calling for a rebate.

Or does it not matter?

For context, I'm wanting to book MAD-MEX on IB metal, then MEX-LHR on BA metal. Never used a voucher in this way before so don't want to mess anything up!