Pace
A very slippery aircraft may mean that at 1.3 there is too great a margin.
The only way to better those fixed figures is to do a flight test on a particular aircraft, work out the correct stall figures in different configs for that particular aircraft and using 1.3 as a baseline experiment above and below that figure
Sorry am I missing something I thought that is what I said.
If no information is given then 1.3 is a good starting point.