TBH, this is exactly why I bought lathe with a gearbox.... the thought of having to deal with change wheels put me right off, maths having never been my strongest subject. When I discovered that my 'box would do metric AND imperial, well, I was a very happy bunny indeed...
I've never really got to grips with how changewheels work exactly. OK, I understand the basic principle, but how do you get the different sized gears to mesh properly, and why is a "tumble reverse" so called? Is that the same as the "tumbler" you mention in one of your posts above?