Match the effective TT length, your CX seat postion will be slightly forward of a road position so result in a slightly shorter saddle-to-handlebar measurements that is just right for cross. Dont go too small on the frame worrying about standover or else you will have too much drop from level of saddle to the bars, wont be able to use the bar drops wich is an important postion for good CX handeling. You might even look at the cross-check in next size up to ensure bar is high enough (so long as the TT is not excessivly longer than your road bike).
It depends on what your intended purpose is. If you like the fit of the Z85, the chart should tell you that you shouldn't get a Cross Check.
The problem with the Cross Check is that the relative proportions of the top tube and the head tube are such that it wants to direct you to an aggressive riding position. If that's what you want, then I'd say get the 54cm Cross Check, but don't overlook the 8cm difference in head tube size between that and your Z85. The fork on the Cross Check is about 2.5 cm longer, but that still leaves your bars more than two inches lower for the same spacer, headset and stem.
What's your REAL inseam (turf to taint)? How tall are you? Do you like the fit of the felt? Can you get up a curb on it? Do you feel like you could maneuver the thing in the way you'd need to on a cross course?
have you ridden the size 56 felt cross bike? What'd you think of that? I'm asking because numerical size is pretty standardized over a brand line and may not necessarily reflect just the BB->TT seattube length, rather a common size for buyers to reference.
I'm 5'10" with a 33" inseam. I want to build a bike for racing, preferably with disc brakes. But I notice that most of the builder frames are multi-purpose, so they have disc and canti mounts, fender mounts etc. Are there any frames for cyclo-racing with disc tabs that are recommendable?
You can get a Specialized Crux with disc brakes. I'd definitely race that. There's also the Felt F65X or the Jamis Nova Race, both of which are at the low end of their manufacturers' offerings but are probably decent race bikes anyway. The biggest problem with disc brakes for racing is that they add a lot of weight to the bike. CX racers tend to be weight weenies.