Since the datasheet that you link to says the flux is a water mixture, I'd say that adding a bit more won't do any harm

If you have a large lump of hard flux, grinding it down using a mortar and pestle will make it easier to make back into paste with water.

I personally don’t like water mixed flux as when heated the water boils off spreading the flux over the job, I mix mine with meths and when this is heated the flux dries in its original shape, very important on small jobs because the sil sol does not spread over the job in hand = neater job = less solder used= less money spent. I have to say that The last bit is a bit of Kieth Hale.

Meths does not ‘burst into flames’. It burns off only after evaporating. Small amounts of vapour will burn with a colourless flame. Alcohol explosions only occur with higher alcohol vapour concentrations in the atmosphere. Stupid people converting fat/oil to biodiesel, or distilling alcohol, in an enclosed space are typical instances of destruction of garages and homes. Particularly when using non-spark -proof kit for the job.

Anyone who has ever used a meths burner or paraffin heater would likely understand how it works.