"after each set of 1M packs are sold a new card will be airdropped to pack purchasers with each pack purchased having a chance of receiving an airdrop card - as well as a chance of receiving ALL of the airdrop cards going forward over the life of the set"

I'm not sure of understanding it correctly: if I buy only one of the first 1M presale packs, how many chances I have to get the cards airdropped every 1 more milions packs sold? 2% as for Doctor Blight?
If yes, is it more convenient for me to wait and buy packs when Vouchers won't be required anymore? Taking into consideration that, at the actual price, I can buy 1 presale pack (pack + Voucher) or 5 non-presale packs without Voucher.

Thanks to everyone who will answer!