Metric systems are not needed for feed rate control and mark setting. Millimeters and inches are an option for displaying information, and are not suitable for controlling iron.
What you really need is a fractional binary number (an unsigned fixed-point integer). By the way, rounding in such calculations is always obtained naturally - approximation to zero. The error does not accumulate!!!
This is a common misunderstanding, related to the error in the difference of two measurements. If we use A and B for the readings, C for the difference, and a and b for the error in each measurement, then
C = (B + b) - (A + a) = B - A + (b - a)
The misunderstanding comes from the mistaken belief that (b - a) means the errors will cancel out and not accumulate when rounding towards zero, i.e. 0≤a<1, 0≤b<1. Yet, this is no more true or false than for any other consistent rounding scheme (i.e, -0.5<a≤+0.5, -0.5<b≤+0.5).
The actual maximum error in C is obtained by considering the extreme cases of (b - a).
For rounding towards zero, these are when (b is almost one, a is zero) and (b is zero, a is almost one): -1 < b-a < +1.
For standard rounding, halfway up, they are when (b is almost half, a is negative half) and (b is negative half, a is almost half): -1 < b-a < +1.
Thus, as long as you use your rounding method consistently, the error in your measurements will stay the same. If you expand the same examination to multiple measurements, you will find that the error bounds evolve the exact same way, too.
However, when we do
mapping, proper selection of the rounding method to be used is crucial. The best example of this is calculating a specific number in the Fibonacci series via
rounding: \$F_n = \left\lfloor \frac{\varphi^n}{\sqrt{5}}\right\rceil, \quad n \ge 0, \quad \varphi = \frac{1 + \sqrt{5}}{2}\$.)
In the case here, conversion from hundredths-of-a-millimeter (or equivalently tens-of-micrometers) to even-ten-thousandths-of-an-inch (or equivalently five-thousandths-of-an-inch) is only bijective when rounding halfway up.