Just get a tape measure, turn the bike upside down, place a pen mark on the tyre tread, place the start of the tape measure on the mark and "walk" the tape along the tread until you come back to the mark. Simple. Some might say you sitting on the bike alters the outcome, but I don't think it matters that much.

If you use the straight-line-on-pavement method, use a small mark of paint on the tread to mark the start, intermediate and finish points.

+/- 1% is about it. Variations occur with differing tyres pressures, differing loads, and as Dutchy says, different ways of riding. And, as the tread wears down (which has more of an influence on altering readings than anything).

Otherwise, if you use the route sheets created by the same people all the time, just increase/decrease the your error by the percentage you have worked out. Which means I don't think you goofed anywhere, although I am slow and would have done this:

Original reading = 2136
1% (or 1/100th) of original reading = 21.36
Subtract from original reading = 2136 - 21.36 = 2114.64

You have rounded to 2115, so we have the same result! Phew!!