Recharging is Step 11 in this thread.
If you have a kitchen scale, that's the best way to measure out the DE. But the cup works. You've done the math right.
PS: After you've recharged the DE and bled all the air out and the filter has run a minute or two, that's your clean pressure. There's a good chance the pressure will not change just from adding the DE. Very little if at all.
Thanks so much again - I just printed it out and I am heading out to take care of it now. If only the dairy farmer down the road wasn't taking care of the cows business today. PHew! Beautiful sunny day here but smelly! Have a good one and thanks again