One rail is 1st/reverse. Another rail is 2nd/3rd and the last rail is 4th or OD and direct. Whether the "fore is a lower gear than aft" holds between the 2nd and 3rd rails is a function of whether or not it's a the pair of gears that constitute "forward gear on the right rail) will either be greater or less than 1:1 depending on if it's an overdrive transmission whereas the rear position on the right rail will always be direct for reasons that boil down to "that's the sensible way to build it".

There's not much to "figure out" when you have a direct lever that goes into the shifter.