Easiwst way is to put the chain on your new set up and check that you can go onto both the largest chainring and the largest sprocket at the same time with about 2 links slack. The chain does not need to go through the derailleur when checking this.
You can use two quick links to insert the extra links you need for the 11-34T cassette, then remove it to shorten the chain for the 11-30T cassette. Then you'll have the right chain length for each cassette, and it's very easy to change between them.
Easiwst way is to put the chain on your new set up and check that you can go onto both the largest chainring and the largest sprocket at the same time with about 2 links slack. The chain does not need to go through the derailleur when checking this.