Objective Test for Running Shoe Cushioning Deterioration

I usually have 8-900 km in pair of shoes...After 100 km I measure the sole depth and when it is 10-20 % less - usually 3-4 mm after 8-900 km - I start looking for new shoes...Very unscientific... but it works for me :-)EDIT:

By the way:

Basically I use a normal ruler. I put in inside the shoe and measure the hight to the next shelf in my bookcase... Easy :-) I do this every 100 km or so.Note that I usually apply a little bit of pressure on the ruler to make sure my custom made insole in properly positioned.Also I don't measure anything the first 100 km to allow the shoe to "settle in" first..

Is there an objective test that I can use to determine the deterioration of my running shoes cushioning? I follow the rule of thumb and replace my shoes after about 500km. But some shoes feel like the cushioning deteriorate faster than others.

Edit: In the answers, outsole and cushioning height is mentioned. While the outsole of my shoe is somehow noticable, how would I measure the cushioning height? From the side and the back, new and old shoe look the same. However measuring depth of the shoe inside, it looks like there is 3mm difference!

My new shoe (0km, bottom/left) vs. old shoe (500km), outsole comparison and heel height comparison. The old shoe cushioning feels worse only after some distance (few km). I am a midfoot/heel striker and run on the road.

Note that the shoes are not perfectly size-aligned on the second image.

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