my god your being precise arent you
set your first gutter bracket pretty tight up to the underside of the slate at the stop end the one before the running outlet at the other end half an inch lower.... a string line from one to the other on the horizontal bottom of the brackets and fix at metre intervals the other brackets to this string line.... I assume your facia is perfectly level if you are such a perfectionist. if it flows then your cooking with gas my friend
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