Even in the late 14th century, we still see medieval fighters wearing a full hauberk under their plate, so what we have is a pourpoint covered by a hauberk covered by plate. For this setup the hauberk hangs down a bit lower than the hem of the pourpoint, just a tad beyond the length of the jupon. The shape of the CdB isn't correct for earlier periods, especially the grand aisette, but if being historical is secondary to being comfortable for your needs, then it's really hard to beat a pourpoint over a more historical gambesson/aketon.
Though, some of the benefit of a CdB might be lost if your maille isn't as well tailored as your arming garment. One of the big problems with cheap maille is the way the sleeves and armpits are constructed. Often more the shape of a slightly tapering tube, it should be formed like the shape of a human arm with tailored elbows and shoulders, attached to the torso a bit forward of directly to the side, and the armpit should ride up where your arm attaches to your torso, not hang down several inches below it. The impact of an ill-fitting hauberk is that it binds when you're moving around vigorously, it's uncomfortable to wear for long periods, and it restricts your movement especially when it's belted or held in place by a breastplate or a Coat of Plates. The real glory of the CdB is threefold: the grand aisette sleeves that allow comfortable full range of motion without the hem of the garment ever moving at all (test this by pointing your finger at the ceiling and looking down at the hem of your shirt), pointing the leg harness directly to a single arming garment instead of having a separate "arming vest" for it, and the compression at the hips that allows the leg harness to ride on the hips and not pull down on the wearer's shoulders. If your maille doesn't fit well all of these benefits will be lessened somewhat, but if you're impression predates a full leg harness then the hip compression won't matter as much to you.
