Yes, test the new cassette with the old chain. Shift into the big chainring and big cassette cog to be sure that the rear derailleur does not "bottom out". With the chain in the big to big combination you should be able to push the upper half of the rear derailleur a good amount before it reaches its forward limit. Shift the chain into the small chainring and small cassette cog. The rear derailleur should not bottom out to the rear. There should be some tension on the chain and no sag in the chain. If this is confirmed then lay the chains next to each other and cut the new chain with the same number of links as the old chain.