Offset path random shape

Hi There,

I'm doing an offset path command on a random shape and I need to repeat that in 4mm increments. I noticed that this command is far from accurate. To be as precise as possible I copy the base shape, paste it in front, apply the offset path command +4mm, +8mm etc (since the shape has round edges I chose round and a miter of 5000), and expand appearance. However, at 8mm expansion the round curves become pointy curves and therefore not mimicking the original shape. How can I fix this or is there a better way to do this? TIA.

AI CS6 latest

OS 10.7.5