@Fry178 Thanks for posting back. I've since upgraded my CPU to a 5900X, which is working fine with some PBO/Curve Optimizer.
The XMP ('A-XMP' in the MSI BIOS) setting in my board doesn't work with these sticks. I had to set the primary timings myself and after contacting Corsair they recommended upping the voltage to 1.37v. That worried me a bit and backed it off to 1.36v which works fine at stock 3200CL16 (and at 3600CL16).
Re. the calculator, yes, I didn't have any reason to doubt it and it was initially just a way to see if I could get the RAM to run at 3600 - just 3600 at all, not aiming for CL16 or anything other than stability. The reason for that was again advice that Ryzen liked RAM at 1:1 with the infinity fabric. I bought the 3200 RAM because at first that was touted as the 'sweet spot' for an R5 3600, IIRC. But of course the R53600 runs at 3600 (1800), so maybe that was just a mistake.
I haven't bought more RAM yet, as I'm not totally convinced that I really need it. I have 16GB but I read lots of recommendations for 32GB RAM - for audio sample libraries in Cubase/Reaper etc. If I can just add two more 8GB sticks and get them to play nicely (i.e. 3600CL16) with the existing pair, then great, but if it's too difficult to set up I'll wait until I can buy a 'proper' set of 32GB 3600 (or maybe 3800) and sell the original 16GB pair.
Seeing as I've changed the CPU I'll go ahead and re-test the current 16GB with MemTest (HCI). I did a shorter test which was fine but, as you say, it doesn't test all of the RAM since Windows is using some.
I have a program called MemTestHelper which sets the values for HCI (I believe anyway). So I'll go ahead and see what happens. The RAM timings haven't been changed and thus far seem stable (they are a bit looser than those in 1usmus's calculator). The MemBench section in the calculator - after an 'Easy' 6GB test - reports latency of 62.1ns, which I believe is okay?