It is not essential to enter a number in the planned admission number box, under School Details (unless the school has a second admission number or a sixth form). If box b is left blank, the net capacity will default to a figure based on the spaces marked as classbases or teaching spaces. If a figure is entered in box b, it is likely to be the current published admission number in many cases. However, we hope that the flexibility built into the net capacity method will enable local authorities to set the net capacity (and therefore the indicated admission number) at a sensible level for the organisation of the school, both now and for the longer term. In some cases, this may involve entering a different (ideal) figure in box b - likely to be a multiple of a whole or a half form of entry (FE). In the example you cite, the planned admission number would probably be 30, although there may be some schools where, for some reason, the ideal might be 32 or 35.
Yes, 16.37 is correct, although perhaps the use of the term number of age groups is a little confusing. This calculation works out the number of pupils admitted in the first year of the school as a proportion of the total number of pupils, so that the indicated admission number(s) can be calculated from the net capacity. In this case, because the first admission year is not the main admission year and is much smaller than the number of pupils admitted in Year 8, the calculation gives an unusually high number of age groups (generally, if the indicated admission numbers shown on the Capacity Calculation page match or are similar to the planned admission numbers entered under School Details, there is nothing to worry about).