Been shimming for a long time. My discrepancy is about 2 extra LOOK cleats based on home measurements. I tried that and it was way too much. Reduced to 1 and now down to about a little over 1/2 a cleat. I slam the cleat as far back as they will go and also use high arch insoles. I also have a very aggressive seatpost angle on both my road and tri bike. (Just anatomically works luckily). I have found I cannot ride SPDs for much over 90 min or else it gets weird. If I wear running shoes mtb ing I'm fine. I also do nothing with my running shoes.

There are some really good threads about whether it's your lower leg, upper leg, or some combo on this forum that I would check out. I also found out in the past some one leg step ups during the winter really helps balance out the legs. Each of my legs look pretty different muscle wise.

As far as I know the only reliable way is with imaging, i.e. a big X-ray. My wife has a 2cm discrepancy from a car accident (smashed femur) confirmed with X-ray (you literally just measure the bone on the image). She uses a custom shim under the cleat as well as a shim in all of her shoes (even her slippers around the house) that is about half the length difference. We are thinking about trying a crank length difference (i.e. a 1cm shorter crank on the short leg side) instead of the shim, but haven't gotten around to buying an extra crank arm and swapping things around.

My discrepancy was measured by the fitter (lay on the table, pull out each leg and compare). Not exact, but close.

FWIW, I have 6mm of shims on one shoe (1/2 of my discrepancy). Prior to the shims I would sit off to one side on the saddle to equalize things (causing all sorts of other problems, ultimately leading to knee pain).

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ECMGN Therapy Silicon Valley:
Depression, Neurocognitive problems, Dementias (Testing and Evaluation), Trauma and PTSD, Traumatic Brain Injury (TBI)

I went and got a bike fit. The fitter said that one knee tracks differently than the other and suggested a shim. We tried it and she said it looked better. She also adjusted the position of my cleats and now I'm scared to replace them for fear of not putting them back correctly. One day I will.