An easy way to determine the useful ISO of an emulsion is to take a picture of the MacBeth color checker or any other similar step scale and the negative with the scale 'centered' in the range of zones is the proper ISO.
In other words, if you get a full range of steps, you have exposed it correctly, but if you get two steps of the same or approximately the same density, you are too close to either end of the scale. Each step of the neutral scale should show a pronounced difference in density.
As for making emulsions with a given speed, it is quite normal to divide emulsion grain sizes into speed ranges, and emulsion content into speed ranges. Therefore, you can normally construct a nomograph of 'average' speeds for any given set of emulsion size / grain content (Br, Cl, I) and come up with a general range for a given emulsion. Superimposed on this is the crystal habit of the grain, with different shape grains giving different speeds.
It is possible to change the speed range drastically by adding ingredients to either enhance or repress the speed for various reason.
When you blend three emulsions, it is generally the middle component which is of most concern to the user (and the film designer), as that is where the image is normally placed in a correct exposure when using negative color and B&W films. Placing the image on the fast or slow component implies either under or over exposure (toe or shoulder). While this is sometimes to be desired, it is not the optimum.
When you blend emulsions, care must be taken to prevent unwanted changes or avoid unwanted mismatches in the emulsions during keeping and coating which can cause changes in their response to light. This can introduce bumps in the characteristic curve.