What is the Sweet point of Design DNI

SAM recommended to use June 21 noon DNI as the starting point. In my case, it is 750 W/m2 (about 85% CDF). Another consideration is to look at CDF% corresponding to the DNI. If I would like to design my plant at 80% CDF, then DNI = 700 W/m2. I understand low design DNI will have higher capacity factor and more solar field area to be used. But using a higher solar multiple (SM) has the same effect as using a lower design DNI. To achieve the same capacity factor and production rate, you can use either a lower DNI or a higher solar multiple to achieve it. So, my question are:
1) what is the sweet point of DNI you recommended to use?
2). what are the best combinations of DNI and SM? I run the parametric model for these two parameters but it does not help. The Low DNI and high SW always generate best outcome. Please advise.
Thank you!
Harry