Having your planks on the diagonal means that they are spanning further than if they were perpendicular, so while 1/2" is generally the minimum for when they are perpendicular, a 5/8" subfloor is strong enough to support tile, and is the minimum I'd want to consider underneath my tile over them. 3/4" is also fine, but the difference may not be warranted, but it would reduce the between joist deflection, but with the planks and the 5/8", you're already above the minimum. Your call, your money. People have indicated that finding a decent sheet of 1/2" is harder than when it is thicker (the thinner stuff tends to be warped, and both cutting it to size and then screwing it down can be a pain).
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Jim DeBruycker
Not a pro, multiple Schluter Workshops (Schluterville and 2013 and 2014 at Schluter Headquarters), Mapei Training 2014, Laticrete Workshop 2014, Custom Building Products Workshop 2015, and Longtime Forum Participant.
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