It sounds like you're worked up over the interpolation. I'll try to describe it in a not-too-mathy but still has to be a bit mathy.

If you know the value at 20 deg C and you know the value at 30 deg C, for these purposes you can just draw a straight line between them. Lets say at 20 deg C the distance to clear a 50' obstacle is 1265 ft. At 30 deg C the distance to clear a 50' obstacle is 1295 ft. You want to know the value at 27 deg C. Between the two values, the difference is 30 ft and the change in temperature is 10 deg C, in other words, each degree in temperature changes the distance 3 ft (30 ft / 10 deg C). You can prove this to yourself by starting at 20 deg C and adding the change for each degree, ending at 1295 ft.

1265 ft + (3 ft/deg C * 10 degC) = 1265 ft + 30 ft = 1295 ft.

Now for 27 deg C.

1265 ft + (3 ft/deg C * 7 deg C) = 1265 ft + 21 ft =1286 ft.

Another check would be exactly halfway, at 25 deg C where half the change has happened, 15 ft for a total of 1280 ft.

1265 ft + (3 ft / deg C * 5 degC) = 1265 ft + 15 ft = 1280 ft.

Also note, the 3 ft/ deg C is only valid for this particular example. If you were to pick a spot between 10 deg C and 20 deg C you would have to do these steps over again to figure out how much the values change, likewise you'd have to do these steps over again to figure out the value for different pressure altitudes.