Example: A 72″ x 42″ x 1″ dining table to be extended with leaves.

Wooden extension sliders are supposed to be 2/3 of the table length when fully closed (72″ x 0.66 = 48″) in order to support the weight and eliminate sagging. 

I’ve been looking at an interesting idea from Jeff Miller in which he uses metal extension sliders for support and stability.

I have a pair of metal ball-bearing full extension sliders (34″ l, 68″  l at full extension, rated at 220 lb. capacity). 

Will these work for the above table? No sagging and adequate support? Is there another guideline besides the 2/3 overall length when using metal extensions? 

Thanks to all for your experiences and suggestions.