I would find it hard to believe that a 1 dKH drop will make that much difference as well, but if you want to experiment and see what results you get:
You can use Muriatic Acid (approximately 33%-35% HCL) bought from a hardware store, you will need to use approximately 0.123 ml of the Muriatic Acid to reduce 1 gallon of salt mix 1 dKH. I would only use half the amount at first, stir your salt mix thoroughly to allow CO2 to dissipate and then test your result to make sure.
So, if you want to reduce 20 gallons of salt mix say 5 dKH, than you would want to use:
20 X 5 X 0.123 = 12.3 ml of Muriatic Acid (approx. 33% HCL) to reduce 20 gal. 5 dKH (Use 1/2 of this amount to start with).
You need to mix the salt mix throughly to remove the CO2 produced from the reaction before you test for alk & add it to your tank.
I would not recommend reducing alk using this method in your tank with coral, fish and other organisms.
Caution needs to be taken when using Muriatic Acid, it will irritate the skin and can burn the eyes. Follow all labeled directions on the Muriatic Acid container purchased. You need to use Muriatic Acid in well ventilated areas. Never add water to the acid, alway add acid slowly to water to prevent possible explosions and sudden release of deadly gases.