Watson, originally from South Queensferry but whose family home is now Elie, Fife, tied with the French 18-year-old Celine Boutier on three-under-par 141, but the Scot's better second round – 69 to 71 – earned her the No.1 seeding.
Having "scrambled" well for her opening round, the Scot, a student at Stanford University, California, was on top of her game. "I hit more fairways; more greens; I played solidly, as I have for most of the year," she said. "Nine of my last 15 rounds have been under par. When I lost in the US Public Links championship last week, the girl who beat me finished with five successive birdies."
Did she feel she had been "wronged" by appparently not being considered for a third Curtis Cup selection in a row? "Not at all. If the people that pick GB&I teams don't think I am good enough, that's up to them," said the former British girls finalist and past winner of the Scottish girls matchplay. "I don't feel I have a point to prove at all to anyone here."
The three who finished next to Watson and Boutier were Wales' Amy Boulden (73, 73), Italy's joint overnight leader Giulia Molinaro (70, 76) and Sweden's Daniela Holmqvist (72, 74).
The only other Scots to figure among the 64 match-play qualifiers were Rachael Watton, of Mortonhall, on 152 (77, 75), Jessica Meek (Carnoustie Ladies) on 153 (82, 71) and Alyson McKechin (Elderslie) on 157 (82, 75). Meek, 17, was on the waiting list until the last minute on Tuesday and her 71 is a record for a Carnoustie Ladies member.
Laura Murray, the Scottish champion, shot 158 (79, 79) but missed out on card comparison.