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A fair amount of slack for a TT bike, access hole to the top tube is quite wide, plus the stem (later covered by a 3d printed top cap that gives that aero finish) has enough room around it, but if you leave too much slack, the outer will push against the 3d printed top cap and the result won't be perfect, last time I opted for some zip ties to hold the outer as close as possible to the stem without compromising steering.
Yup, that was the same issue with the Canyon up above. mix of 9150 and 9160 one button silliness. That was all a doddle, till it came to rear brake routing. Whole brake assembly has to come out to get access to battery and cable guide.
How much slack do you get on this? Canyon didn't offer much.