This one seems a bit more straightforward than how some of the other sets effect things. My understanding is that normally you have a 1% chance for any legend that drops to be eternal and 1% for it to be crystal. 1 in 100 chance for either. When you wear Crystalline, you get 50% bonus per rank...so at Crystalline(6) you get 300% more chance, which is a 4% chance for a legend item to get a crystal stat, which is 4 in 100 or 1 in 25.
As far as the crystal quality drop bonus, that one is a bit more confusing to me, but here's what I think...I believe there are 2 different calculations that go on for crystal drops. There is the range of crystals that might drop based on what floor you're on, so that is determined first. Once it has the range, it rolls to see what crystal quality you get within that range. I believe the 7.5% bonus per Crystalline set gives a bonus to that roll (along with the bonuses on legend and eternal maps) to bump up the crystal quality within that range. The thing I'm not sure of is, how that roll works.
Knowing how other games work, my guess there is, there's a 1-100 roll and the bonus is applied to that roll. That roll however, only yields access to part of the crystal range, so lets say you're above floor 500 and you're rolling for Kyanite and beyond. I'd guess that the main range is Kyanite to Amber, and that to get a crystal beyond that, you have to roll a 100, (or maybe a 95-100?) and if you do, you get a second roll to see what crystal you get from Jasper to Obsidian (although with how rare Ruby, Amethyst and Obsidian are, it could be that those 3 require a second 100 roll.)
Anyway, those are my thoughts. I'm not a dev here, so obviously they are just guesses, however I've coded for other games so I have some understanding of how these types of things might be coded.