I have this exact issue, and came here to post about it, but I'll just piggyback here since you've already explained it.
I have the JDL kit, but I have an automatic transmission so instead of my BOV pulling vacuum from the brake booster line, I have it hooked up directly to the intake manifold. I was getting turbo flutter on the spring the BOV came with, so I bought a pack of two inner springs (-4 inhg and -8 inhg) and one outer spring (-15 inhg). I still get flutter with the -8 inhg spring, even though this converts to -3.92 psi and my idle vacuum shows anywhere between -6 to -8 psi. When I tried the -4 inhg spring, the BOV is open at idle. So this makes me think there is an issue with the connection between the IM and the BOV, but I've checked it multiple times and all connections are secure. Could it be that I need tubing with a larger inner diameter?