Fit Experts Please
 

I need some advice on sizing a new cx frame
I'm very good at fitting mtn bikes but cx I'm not as good
I'm 5'8" with a 31.5" inseam
I'm familiar with the effect seat angle has on seat position, reach, and effective top tube length.
Here's my question, based on my size and a seat angle of 74 degrees, what would be a good Effective top tube length for My size?

My xc mtn bike has a 73.5 degree seat angle and a 23" ETT length which is perfect with a 90mm stem.

I plan an racing it SS cx, and using it for SS road training.

Thanks for any recommendation

Dave