Measuring sit bones--is my yoga mat defective?

[Ana]

I am having the worst time trying to measure my sit bones. I have tried the paper/yoga mat but my sit bones don't make a deep enough impression to measure. I even made my own butt-o-meter with a ziplock baggie, water and flour but it's still very unclear where my sitbones are. I can feel them with my fingers when I'm sitting but they won't make impressions on anything.

I've done the butt-o-meter at the shop but it's far away and inconvenient for me so I know my sit bones can be measured.

Does anyone have any other tips?

I'm trying out the old Jett model (with seams) in 130 which is my previous Specialized saddle size. There is no soft tissue pain but a lot of weight put on my sit bones, even with my most heavily padded shorts. The pain is not as bad as the Terry Butterfly I tried a while back (I had to stop riding after 20 minutes my sit bones hurt so badly).