Drag separation - proper fit of ISC

I've been posting questions along the build of the Terrier/Sandhawk I've been building and I'm almost ready for a test flight. Today I put the stack together in a "flight ready" configuration and found the fit between the coupler and the sustainer a bit snug. It comes apart with a little effort, but isn't exactly loose. As opposed to the booster end, which has a lot more play in it (will need to snug up with tape).

So here's the question: This kit and configuration depends on drag separation. With that in mind, how loose should the upper stage be? My current concern is if the lower end of the inter-stage-coupler is looser than the upper part, that's where it will separate - instead of the upper end where needed.

Thinking maybe a shear pin on the booster to keep that connected until ejection, but I'm planning on doing motor eject on the booster and unsure if that will be enough of a charge to break the pin?

I use shear pins on booster to ISC. As far as the sustainer to ISC fit, it should be pretty loose. You should be able to easily lift the sustainer from the whole stack with no effort. I also put a little baby powder on sustainer coupler.

Thanks this is what I figured. I'll keep sanding... The ejection charge calculator I use seems to indicate I should have enough pressure with the 1.3g of 4F bp in a CTI reload (tho I may be using an AT DMS for the booster - assuming it has about the same amount of bp). 14.85 psi - 2 2-56 pins max (I'd probably just use one)

I also use a 0.5 gram BP for a ISC separation charge should drag separation fail. It can happen.