aphil's approach is the best way.
FitDay has lasagna in the database, and you can view it by size. For example, for "lasagna, with meat and/or poultry" a piece approx. 2-1/2" by 4" is said to have 328 calories. A piece approx. 3-1/2" by 4" is 370 calories. But of course it depends on the ingredients. FitDay has many different types of lasanga listed.
It depends on how close you want your count to be.
MindiV, if your husband doesn't like the lasanga cut up into pieces, how does he eat it? Just sits down with the pan and a spoon?
Jay