You’re overworking our “trick” for adding octal numbers. You added 38 and 28 and got 78. The “trick” of adding 2 to the decimal sum to get the octal sum is good only for octal sums from 108 to 178. Since the octal system is identical to the decimal system for sums of 7 or less, you don’t need any special rule for sums of 7 or less. 38 + 28 = 58, just as 310 + 210 = 510.
