Please use table

http://www.surface-tension.de/

Maybe a lower concentration solution for spin coating process can be better spreaded on substrate.

Have you tried using a two-step process? A slow spin to spread out the PDMS then a faster one to achieve the desired thickness.

Could i check your substrate size ? if you do not care of the size, then may be use lower concentration solution with drop casting tech onto a pre-treated smaller sized substrate.