Ah, this could be tricky. Somewhere I have the original nut that my CK35 came with and maybe I might be able to measure the spacing on that for you. IIRC the nut is 35 mm wide and the strings were reasonably spaced out, say 28mm g to A. I like my strings to be a bit further apart than that and wasn’t certain what material the nut was made, I made a new nut out of bone and replaced the original nut - just to be sure of what was fitted I made a replacement saddle out of bone too. Maybe ask a few other people for now, if you can’t get an answer off of anyone else then send me a pm in a few weeks time and I’ll try and find the old nut.
The CK35 is quite a nice instrument; mine sounds good now but to get there it need some set-up work and to open up a bit. As for the SK25; if one crossed my path then I’d give it a try and hope I’d got a good one, or at least one that I could sort out. There was a thread sometime back where a Canadian Lady said how pleased she was with hers, but I accept that it might be a bit hit and miss. Some dealers are more able to and more active at weeding out the duds than others ...