1d10x10 is very easy, yes, but unfairly scales, IMO.
RAW, if you teleport 80 miles to the neighboring duchy, the most you can be off target is 80 miles (and that is not likely; more likely to be less than 40 miles off target). 1d10x10 miles would make a flat distribution of equally likely outcomes that could send you 10, 20, 30, etc., up to 100 miles off. Also, if someone wants to burn a teleport spell to travel just one mile, I don't see any value in a mishap taking them more than a mile off target.
RAW, if you teleport to the other side of the planet (assuming an Earth-sized world, that would be either 7900 miles or 12500 miles, depending on if you see teleport taking a straight line to the destination or not), you could end up 7900 miles or 12500 miles off, and even a small error of 1% would be 79 miles or 125 miles off target. Using 1d10x10 would generate error distances between 10 and 100 miles, same as above. I like large distance teleports to be risky, and using 1d10x10 miles negates much of this risk.
