As much as I like RPN, no help on this one. I was thinking you could use the stack to store the intermediate result (target size x 25.4) but that doesn't work.
So RPN or algebraic, you would store that result in a memory, and recall it for each ranging. Assuming all the targets are the same size.
So with a course with multiple targets of all the same size and using a calculator, formula A allows you to store the intermediate result and not have to do the whole calculation each time.
This is a good point. target size x 25.4 is a useful number that shouldn't be calculated more than once. It's the target size in mm. You should be thinking in metric if you want your results to be in meters. target in inches/ mil, on the other hand, is sort of useless on its own, which makes it tougher to get a grip on mentally.
So, in m head, it's really more like this - two separate calculations:
1) target in inches x 25.4 = target in mm
Done with part 1. From now on I know my target in mm. This has a benefit of also having a sanity check. You can look at the target and verify that you did the math right because you know what 550mm should look like.
2) Next calculation. target in mm / mils = range in meters.
If I do it again at a different range, I'm reverting to the metric target size and starting at step 2, not going all the way back to the imperial one.
So I guess I change my answer. I break it up into two calculations mentally. But if you insist on having one big equation, it's the same.
The concept here is to keep the numbers relating to something physical as much as possible to help stay mentally grounded and to keep things verifiable via simple observation.