I have the probe almost directly on the heat tape. I have it separated by a thin plastic "o-ring" from a drink, which sets it off a few mm. I also have it covered with cardboard/tape so that drafts don't artificially cool down the probe. Currently my herpstat 2 is set to 99 degrees which gets a good range (hotspot of ~95 inside plastic) on most of my shelves.

It's not clear to me how relocating it to another shelf will affect much, except causing me to potentially recalibrate by a few degrees but ultimately delivering the same average power to all shelves. I'm willing to try it if someone can provide a good reason.

I did think of one thing I could do on the hotter shelves which might work. Replace the shim with something taller, and/or something thinner. Taller to get it further from the tape. Thinner to conduct less heat -- I'm thinking the shims may be what are creating some of my hotspots, since they make direct contact.