I started watching the totals GDXJ held in FF shortly after the announcement was made about the rebalancing and I know at one time they held 87 million shares (they may have held more, I don't know), so they've sold down to what they currently own.
There have been various posts on this board about what they'll end up with and of course I don't think anybody knows for sure. To this point I don't know if GDXJ even knows what the final number will be. Some days they sell shares other days they may add a few shares. I don't know the method to their madness.
Anyway, we know they can't hold over 20% of the total FF shares outstanding. The review of the rebalancing with happen on June 9th and I expect in that next week GDXJ will sell what's required to bring their FF holdings in line. The GDXJ selling is supposed to end on June 16th, so any adjustments should be made from the 12th through the 16th.
You could very well be right with your prediction of what they end up with that I don't know. It will interesting to see what happens in the last week of their supposed selling. That week could be very volatile for sure.