I have a quote from a cycle shop for a new (shimano) chain & cassette if required due to slipping in the top 3 gears. Cassette £55, chain £35 and the labour to fit the pair £25Don't know what your reason might be. Order a new cassette?
Maybe you want to work out the gear inches, in which case there are online calculators.
That sounds like the perfect cassette. Always having one more gear to drop into is the ideal.