Hello. I'm also in the middle of a SPP Harvester trial and am...

Hello. I'm also in the middle of a SPP Harvester trial and am thinking of going about it myself through a cheap brokerage by selling down bit by bit to 1 share per company. Does anyone know if there's anything special about the 300 chosen apart from being the largest ASX listed? Is there a way to determine what companies are more apt to put out a SPP? If anyone has any tips on this, thanks.