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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.
R9150 this time, they are fairly easy to build, as long you have the right tools (internal cabling guides).
Trickiest part is getting the brake outers through the bars and into the stem cover/frame.
There is no room for slack but you need to be able to let the right length for steering and handlebars removal for transportation/packing.