The formula you mentioned represents the axial load capacity of short, axially loaded reinforced concrete members. The constants (0.4 and 0.67) are derived from experimental data, but if you change the concrete contribution from 0.4 to 0.5, the concrete’s contribution to axial load increases, which might lead to a higher calculated capacity, but it could also overestimate the actual strength, especially in lower-strength concretes. Similarly, changing the steel contribution from 0.67 to 0.6 would reduce the steel's contribution, potentially underestimating the member’s capacity, especially for higher-strength steels.

These numbers are not arbitrarily decided. For axially loaded columns, the ultimate load is the summation of loads shared by concrete and steel at the failure strain limit of 0.002. For details, refer SP-24, clause 38.3.