I think you need to approach this by thinking about the precision in cell or grid sizes. For instance 4QFJ 12345 67890 .......precision level 1 m would give you a 1m x1m cell within a grid. So I believe that both these coordinates 4QFJ 1234 6789 to 4QFJ 1230 6785 will give you a cell that is 10m x 10m. If you are after 50m* 50m blocks then the format you have should achieve this. I think the issue rounding up values is that it can put you out from meter to a few 100 meters depending on the precision. eg if you have decimals in your UTM coordinate of 55 K 428252.56 m E 7782667.61 m S and by rounding-up, it puts my MGRS coordinates out by a meter (55KDT 28253 82668) when the real MGRS coordinates should be 55KDT 28252 82667.