If we assume two things, one that it will work like it did going from A Realm Reborn to HW, and that 270 (for armor, 275 for finished Anima Weapon) is the highest ilvl of HW...

130/135 was the highest for ARR, 115 was the lowest in HW, this would mean Stormblood gear starts at 245, rising 5 ilvls per level and reaching 270 at 64 (275 at 65 for Anima Weapon) and Maxing out at ilvl 300 from the first Level 70 dungeons.
First tomestone gears (ones bought with Allagan Tomestone of Law) were 10 ilvls higher than the gear from the first dungeons loot (Neverreap and Fractal Continuum) so Stormblood's would be ilvl 310.

If we assume 3 new sets of Tomestones, ones that worked like HW, their Ilvls would be 340(350 upgraded), 370(380 upgraded) and 400(410 upgraded).