I've downloaded a calculator to work out loads depending on series or parallel arrangements and also give the total travel until it bottoms out. This is where the tricky bit is to ensure the loads and travel all work out.
I had allowed for 10mm total travel of the drawbar to release the tool, of which 6mm is used to remove the pre-load off the taper, and the remaining 4mm is to push it out and hopefully release the tool.
Thanks for the info. From what I have read (and I am no engineer) a R8 collet requires a holding force of about 2400 lbs to be safe. I am using part No. D2315162 washers from Belleville Springs 2 in P and 6 in S this give a free stack height of 28.5mm and a flat height of 24mm. 75% deflection height is 25.13 mm with a load of 12337N.

The guy on the zone recons 0.5mm will release the collet after the pre-load has gone.
The setup requires about 180Kg on the jack to give me 1260kg

I would be interested in the link for the calculator