The method depends on what you have there. If you have a small kit then that is the ratio 1 to 1!
If you have 250ml of the main adhesive, then you have 15mls of hardener?
Sooooo, you can divide the 250 ml up into say, half, and then half the hardener. If you are really picky then use a 10ml syringe
for the hardener.
I did this last week - worked a treat (PVC two part)
Another way would be to ask Poly. He'll probably tell you a LOT of stuff and then suggest that you take a known (measured - lol, try measuring glue) quantity of adhesive and multiply by 6% = quantity of hardener. Something like that