Reach as published is a frame specification, not a finished bike specification. There is also the idea of finished bike reach, but numbers like that are never published as part of frame geometry. Your number is frame reach and has nothing to do with stem length.

Also because of the angle the stem the length doesn't convert directly to reach. The effect on reach is a little shorter than the length of the stem.

One thing you can do is Google "bike stem calculator". You will get a web site that compares th effects of different length and angle stems. Also takes into account the effect of spacer stack.