Yup - combining temps need to be the same, no matter what temp you choose. We used a candy thermometer to make sure.

The calc table we used is now gone, but would have been much the same as what you used. We were aiming for 5-6% superfatting with the lard and 11% with the beef tallow.

However, the page where got the recipe (ratios of fat:lye) is still there and - duh! - it's called laundry soap! Why didn't I see that when we were making it? No wonder it seemed too harsh for hand soap even with the extra fat! :)