Minor Loss Coefficient, K

I'm working on designing a new pump station and in my analysis I ran across a burning question. In textbooks, pipe, and pump manufacturers literature they provide minor loss coefficients, K, for common fittings. Where do the K numbers come from in the tables? I ask because I want to know what the K values are for a 22.5 and 11.25 bend and a plug valve. How would I go about getting the K value for reducers? On the projects I'm working on I have a 12"x8" reducer and 8"x6" reducer and don't know what the K value is so that I can get the equivalent length. I know in the future I will run across this again as I select different size reducers.

A colleague I work with uses slightly different method, L/D. Those values are different than what you see with K values. Even with that method I still don't have all the values I need. I have values for 90 and 45 degree bends not 22.5 or 11.25. L/D tables we have do not cover reducers either. Have any of you created a spreadsheet where you calculated your own K or L/D tables? TIA. 

My best guess would be that the K values are calculated using the minor loss coefficient based on experimentally determined head loss. (but don't quote me) 

You might be able to get K values from a given manufacturer or supplier of the parts you're using. Without that, you can probably get pretty close using some assumptions.

One way to do it would be to just treat the small bends as 45 bends, which would give you a conservative estimate (since you'd be overestimating friction in each smaller bend). I don't expect that the difference would be too huge (Though it depends on how many there are, and how long the run is).

Another option to get a little closer would be to look at something like this table, http://www.metropumps.com/ResourcesFrictionLossData.pdf which provides loss coefficients for mitre bends.

Regarding the reducers, typically a supplier will have a K value, IF NOT, there are some formulas which can get you these values, though you'd need some other information about flow regime/pipe friction (reynolds #, friction factor)
 https://neutrium.net/fluid_flow/pressure-loss-from-fittings-expansion-and-reduction-in-pipe-size/.

Hope this helps!