Can anyone advise me how to decode the LR value, or use it in my 70D?
You can't, really. To explain why requires a nut-shell description of digital imaging: The sensor registers only the intensity of the light striking each pixel. That light is either red, green or blue according to the micro-filter in front of it, but only one of them. The data at this stage, with only one value per pixel, can be siphoned off as a Raw file for later processing or used immediately by the camera in its jpg processor. Whether the processing is done by a Raw conversion program or by the camera, it starts with the same first three operations, but the details of the way these operations are performed are different. The operations are (a.) demosaicing, (b.) profile assignment and (c.) white balancing.
Demosaicing involves interpolating into each pixel the two other values needed to make an RGB color image. It is done by a somewhat complex examination of the neighboring pixels to calculate what those values should be and the results can effect not only the colors but also detail extraction, sharpness, noise and the smoothness of transitions from pixel to pixel. There are scores of algorithms for doing this that have been designed and Canon and Adobe use different ones.
Next a camera profile - a mathematical model that essentially is supposed to describe how the sensor reacts to different colors in specific light environments and make corrections for deviations from the scene colors - is applied to the color data set. Canon has this information because they, after all, designed the sensor and its micro-filters and so their profiles are quite accurate. Adobe has to test a unit or two of each model by shooting color targets under controlled lighting and building the profile. The result is that the profiles are different.
Afterwards comes WB, which involves manipulating the RGB values so that the color cast from the lighting is neutralized. Thus WB is a set of linear multipliers applied to the color channels. The camera and LR arrive at these three multipliers in one of four ways: Auto WB, in which the scene colors are averaged and the multipliers needed to make that average neutral grey are calculated; Custom WB, in which a grey target is examined and the necessary multipliers calculated; Preset WB, in which preprogrammed multiplier values are used; and Temperature setting WB, in which there are preprogrammed multipliers used according to the number set. However, when a custom WB is calculated (in LR by using the eyedropper, in the camera by examining a photo of a grey target) it is done on the basis of the colors already produced by steps (a) and (b), demosaicing and profiling, and so the multiplier numbers are different.
In its UI LR does not show you the multipliers it is using. Instead it translates them into Temperature and Tint numbers. The temperature roughly corresponds to the illumination for which those multipliers, applied on top of that profile, would produce a neutralizing effect. (I say "roughly" because color temperatures do not correspond to one certain color, but rather a range of colors). In the camera, on the other hand, if you set a Kelvin number, the multipliers that are used are those that the firmware designers calculated would produce neutralization on the basis of the Canon profile. If you set the camera to 5000K, for instance, it will use multipliers x,y,z for making jpegs and write x,y,z in the metadata. LR will use x,y,z, but because it is using a different profile will translate them to a different K number, 4700K maybe. And going the other direction is impossible because if you take LR's K number (4700) and dial it into the camera it will cause the use of a different set of multipliers - x1,y1.z1 and not the x,y,z that you want.
As for the Tint setting in LR, who knows what it means? It is a purely arbitrary scale of -150 to +150. The camera's WB bias function at least gives values on the green-magenta axis in decamired units, a recognized scale, but how can you translate from one to the other without knowing what shift LR's numbers represent? And the Tint numbers, although they work primarily on the green-magenta hue shift, are not simply that because changing them causes changes in red and blue also.
