This thesis consists of two different studies in the field of precision calculations for elementary particle interactions in QCD. In the first study we developed an automated algorithm to calculate one-loop jet functions. The algorithm has been implemented in the Mathematica package GOJet. By using the geometric subtraction scheme, we regulate the soft and collinear singularities in order to render jet functions finite. The package allows one to calculate jet functions for generic IR-safe observables at one-loop order in QCD. In the second study we calculated, for the first time ever, the doubly fermionic nf2 non-singlet coefficient functions C2 and CL for inclusive Deep Inelastic Scattering (DIS) at order N4LO in the strong coupling constant αs. By using the optical theorem, the integration-by-parts technique and the method of differential equations, we were able to recursively compute a sufficiently large number of Mellin moments (N = 1500), which allowed us to reconstruct full analytic expressions in both Mellin-N space and Bjorken-x space, in terms of harmonic sums and harmonic polylogarithms respectively. We verified the CFnf3 leading large-nf contributions to the coefficient functions CL(4)(x) and C2(4)(x). On top of what we agree with the coefficients of the large-x plus-distribution in C2 and with the resummations of log(x) and log(1 − x). As a byproduct we verified the N3LO nf2 non-singlet splitting function Pns(3)+(x). Furthermore, the no-π2 theorem allowed us to predict the ζ4-terms of the 5-loop non-singlet splitting function at order CFCAnf3 and order CF2nf3.