36 of 100: Add Seven Subtract One

$\Large 1 \; \square \; 2 \; \square \; 3\; \square \; 4\; \square \; 5 \; \square \; 6 \; \square \; 7 \; \square \; 8 \; = 30$

Six out of the seven "$\square$"s above contain addition signs, and the remaining "$\square$" contains a subtraction sign.

Where should the subtraction sign go to make the equation true?

Can you solve this puzzle without using guess-and-check at all? Strategy hint: what would the sum be if all the squares had addition signs?