K is determined by using a torque transducer, load cell, and the other hardware like washers and nuts that you are going to use in your joint. You want to make sure your setup is as close to actual production as you can get. You then tighten to 75% of the bolts proof load as measured by the load cell while also measuring torque. That gives you enough information to use the formula to determine K.

K = T/(DP)

T = Torque achieved at 75% of bolts proof load

D = Nominal thread diameter

P = Proof load of bolt

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Using our example above:

111/(.250*2025 = ~.22

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This would be an example setup from work:

https://pieng.com/testing-overview/torque-tension-testing/