  Recent studies classify the topology of proteins by analysing the
distribution of their projections using knotoids. The approximation of this
distribution depends on the number of projection directions that are sampled.
Here we investigate the relation between knotoids differing only by small
perturbations of the direction of projection. Since such knotoids are connected
by at most a single forbidden move, we characterise forbidden moves in terms of
equivariant band attachment between strongly invertible knots and of strand
passages between $\theta$-curves. This allows for the determination of the
optimal sample size needed to produce a well approximated knotoid distribution.
Based on that and on topological properties of the distribution, we propose a
numerical measure for the determination of deeply knotted proteins that does
not require the computationally expensive method of subchain analysis.
