Some measurements are more forgiving then others and it also is dependent on spoke crossings. As example- A 4 cross spoke leaves the hub at almost a tangent so it's length is minimally effected by flange diameter. But a radial laced spoke is VERY dependent on flange diameter. Skinny spokes can stretch a mm where as thick spokes have little stretch. Even the nipple contours as they seat into the rim can change the working spoke length. Getting a spoke within a couple of Mms of it's best length is close enough in my book.

One technique that I've used is to round each factor up then the next one down. Run the calculation. Then go back and reverse the rounding so the rounded up factor is now down then re do the math. If the results are far different remeasure and re calculate. Andy