Tips if you just want to mix part of a bag of tile adhesive

[markharro]

I probably only need to mix up a 1/5 of a bag to use to stick hardie board to masonry at the back of our shower enclosure. Do I need to weight it and then accurately add the correct amount of water pro rata or is this overkill?

 

Also will the rest of the bag contents go off once opened as I wont be using it for the main tiling for maybe another 2 months!?

[Russell griffiths]

Put the guesstimated water in a bucket, add the powder,whisk with a power drill, put the half bag in the corner for later use. 
Easy as that.