A measuring cup may weigh liquid volume, but not solids. For example, a cup of shredded cheese generally weighs only 4 ounces, while a cup of water weighs 8 ounces. You'll need to weigh the velveeta. Or, if you don't have a scale handy, mark the block accordingly. (Score an 8oz block into 8 sections, and 3/4 of the block is 6oz, ...etc)