The dose math (and why most guides overshoot)
grams of baking soda = 6.8 × (gallons ÷ 100) × (ppm increase ÷ 10)
That figure comes from straight chemistry: raising alkalinity 10 ppm in 100 gallons needs about 6.4 grams of sodium bicarbonate, and the pool industry's working number of 1.5 lbs per 10,000 gallons per 10 ppm lands at 6.8 — the two agree within a rounding error. The advice you'll find repeated across most spa sites, one tablespoon per 100 gallons per 10 ppm, works out to roughly 16 grams. That's about two and a half times what the water actually needs. In a pool the excess disappears into the volume; in 350 gallons you feel it, and you end up fighting high alkalinity instead.
A worked example
A 350-gallon spa reading 60 ppm, heading for 100: that's a 40 ppm rise, so 6.8 × 3.5 × 4 ≈ 95 grams — about seven tablespoons, or a bit under half a cup. Add half of it, run the jets for 30 minutes, retest, then decide whether the rest is needed. Spa water responds fast, and the only way down from an overshoot is dilution.
Alkalinity before pH — the spa-specific reason
In a pool, alkalinity-before-pH is good practice. In a spa it's close to mandatory, and the reason is your sanitizer. Dichlor and bromine are both acidic, so every dose nudges pH down. That happens daily in a small body of water with a lot of aeration from the jets. If TA is sitting under 80 there's nothing absorbing that pressure and pH swings constantly — you'll chase it with pH-up twice a week and never win. Set the buffer near 100 first and pH mostly takes care of itself. The hot tub chemical calculator grades all your readings together and puts them in that order.
When alkalinity is too high instead
Above about 120 ppm you'll see the opposite problem: pH creeping up, cloudy water, and scale forming on the waterline and heater. At spa scale the honest fix is usually a partial drain and refill rather than dosing acid — 350 gallons is a few dollars of water and about forty minutes, and it resets total dissolved solids at the same time. Chasing high alkalinity down with acid in a small tub tends to overshoot pH instead.