The material you are biting on seems too rigid and the surface area too small for your dental arches. I measured my original intermolar width by biting on a block of cheese, ha. Worked adequately.

This might give you a rough approximation. Have you had any extractions? You'll have to work out which of the dents corresponds to the cusps of your first molars and then measure the distance between them. You might subtract a mm or two since the gingival border of the molars will be slightly closer together than the cusps.

I put a bit of paper on top of the card. I start at the front of the mouth, identify the  first two pointed impressions, the bicuspids, then the next pointed impression is the foremost inner cusp of the first molar. It is surprisingly far forward in the dental impression and in my case a lot narrower than the distance between the molars at the back.