WATs app
Yes on the face of it the figures seem contradictory but remember that in very high ambient temperatures, on short or limiting runways and at high altitude airports the Trident was unlikely to be able to lift MTOW anyway. So the extra 4% or so of thrust was reflected by the increase of the limiting (wet) take off weight compared to the dry limiting take off weight.
In practice that was enough to lift the water and a bit more payload if the system worked. In practice it often didn't and in my book I want to describe how we had to return to stand, drain the unused water and; not only offload the few extra passengers we thought we could carry, but also a few more. Because the air temperature had increased whilst we'd been farting around with a system that wouldn't have been necessary, if BEA had let DeHavilland build the aircraft they designed in the first instance viz. the Airco 121 with Medway engines alias the Boeing 727 but that's another story!
PS The 658 Kg of water was soon dissipated (if the system worked) as was the resultant extra 4% of thrust. Then the fun really started.