I got my first custom bike because the top tube length on the closest production model to what I needed, according to my fitting, was 3cm too short with the longest available stem. The bike I was riding at the time had a 14cm stem, long reach bars, and was still 5cm too short.

So not even close. The only dimension on the bike I fret down to .5cm is the fork offset, and sometimes the stem. I have had a couple custom stems to tidy things up.