Gearing for 1×9 cx

Putting my old commuter (Roadrat) into cx guise for the odd race. I either leave it ss (but put on a smaller chainring, probably to 38×16) or make it 1×9. I have an SLX 12-28(ish) cassette somwhere but can’t decide on chairing size – any advice? Possibly stick with 38?

I ran a 38 ring with an 11-27 cassette last year and it worked well. This year I’ve gone 12-27 10speed with a 42 front ring, slightly bigger gearing but really nice spacing ratios.

Ta for that. I’ll try the 38 and see how I get on.

38 x 18 SS worked for me