With transistors off of the same reel, the difference that I have measured between parts in this circuit is extremely low - non-consequential for my purposes. So given no thermal differences in transistors, I am happy with both absolute and relative accuracy. My only concern is the different current between parts that are at different temperatures.Temperature differentials across the board ARE a temperature mismatches. It can be characterized two ways. The currents in the mirrors are not the current that is being programmed, or the currents in the mirrors are not the same as each other. Which is more important? One way to get at this is to ask yourself which is worse - that the current, on average, in the mirror is within 10% of the programmed current but that there is only a 1% range between min and max, or that the current, on average, is within 1% of the programmed current but there is a 10% range between min and max.