Extra suit layer and weighting

[mattcassell]

I dive in a Mares Isotherm Semi-Dry that I got back in December. I am finding though that it isn't quite enough to keep me warm when the water temp is below 55 (I get cold fairly easily). My wife picked me up a 3mm shorty a few weeks ago and I have yet to try it out. I'm going out on Saturday and I was planning on wearing it under my normal suit (I have tested it and it is surprisingly comfortable - at least on land) because I expect the water to be in the 40s and I am planning on doing 2 dives that day so I don't want to get too cold.

My question is, does anyone know the approximate weight that I'll need to add to compensate for the extra layer?

Thanks

Matt