the shotglass method will work for now to test FC, keep the FC at shock levels as consistently as you can and keep filter on 24/7...
Yes, keep your water level filled when you vacuum- if it is starting to clear, you can possibly even vacuum through filter instead of to waste to conserve water/chemicals...
DE is Diamataceous Earth (spelling?), you can add a cup at a time to your skimmer, wait 15-20 minutes and check pressure on your filter- when your pressure raises 1 psi, you've added enough- it fills in gaps in your sand, creating a finer filter media to filter out more suspended crud from your water; then it will be removed along with the crud when you backwash, so you add it again, the same amount as you determined the first time by the cup method!