The way I do it - and it's not the only way - is to use a 1" block of wood (not 3/4", needs to be one full inch) and sit the bracket upside down on it. With the bowl sitting flat on a table upside down, (rim on the table) slide the bracket to the bowl and mark the holes. If you want to get fancy, cut the 1" block to match the curvature of the bowl. Same for the other side, but take time to be sure it's exactly on center. Not sure which brackets you are using, and mine may differ in some way. Bottom line, measure twice, drill once.
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Marty