Porcelain Paver on concrete slab help

[antsals]

Evening all,
Need a little help / advice. I decided month back...last October that the best way to lay my new patio with porcelain paver was tile adhesive on a concrete slab. I was working on a 1:60 - 1:80 fall. Aco drain installed etc..The area is about 35m2...first time I've laid this size of slab....

Just checked levels today and I'm 20mm short in one area, what can I do to fill the void? I'm used to using self level, but is there an exterior type that can be installed at a fall? Also how would I ensure I maintain the fall across 3.6m?

Appreciate the help / advice.
Cheers
Ant