...
The lat/long equivalent of T639 is 0.28125 deg (360/(2*(639+1))). However, the GRIB1 format only supports three decimals, so we recommend you in any case round the resolution; we recommend to at least 0.3 25 deg. Specifying a higher resolution is technically possible, e.g. as 'grid':'0.1/0.1', but this only oversamples the data and does not improve the accuracy of the data.
...