The higher CH recommendation compensates for the lower TA recommendation. We recommend lower TA for SWG pools to slow the faster pH rise typical of SWG pools. By using a bit more calcium, the calcite saturation index (CSI) will be closer to zero than at the lower CH recommendation, which helps protect the plaster. After you enter your results in PoolMath, you can see the CSI result at the bottom. There's every chance you'll be fine at 300 and keep in mind that your CH is likely to rise a bit due to evaporation and refilling with water that has calcium. For plaster pools, a slightly negative CSI is a good place, so -0.2 to -0.1 is a good range to shoot for.