Wow; yup, 6.4 dkH is definitely low. Otherwise, your calc being high will help you more than hurt you in this case. For the reason that high Calc doesn’t hurt much, I’ve always been a fan of keeping it on the high side.
Keep raising the Alk, and testing both. You can calculate how much Alk to dose (depending on the concentration of product you use). In your case, I would recommend not raising it any faster than a maximum 0.5 dkH per day, but maybe 0.25 dkH after the first 1 or 2 days.