The spring does NOT just control when the vacuum adv starts coming in, it controls how much advance the can adds per the engine vacuum applied. If You're cruising at a steady speed & the ported vac is 13" on a slight grade, increasing the spring load adj. will reduce how far the can arm moves, and the °adv will reduce as well.
DON'T get all mixed up with the total timing You've already established, leave that alone, the total with vacuum is independent and needs dialed in as such. It will be a variable total at all part-throttle operation above the basic total You've set. What works best will depend on carb calibration, operating temps, burn characteristics ....too many to "guestimate". Trial & error aka test & tune are the only path to nailing it. Have fun!
