Parts get cut to the wrong dimensions because you’ve added up measurements incorrectly.
Get a fractional calculator — an electronic calculator that lets you work with fractions instead of tenths, hundredths and thousands of an inch. This kind of calculator lets you add up numbers and get the answer as a fraction. You don’t have to remember the decimal equivalents for fractional numbers (how many thousandths is 2-1⁄32"?) and then convert them back and forth — another source for measurement and calculation errors. Sometimes sold as “builder’s calculators,” even the most inexpensive models allow you to add, subtract, multiply or divide fractions with mixed denominators (11/16 and 3/4, for example) and display the results as fractions, typically down to 64ths of an inch.