The charge simulation technique (CST) was applied for computing the electric field in long space-charge-free rod-to-plane gaps with G (=gap-spacing to rod-radius) ratio in the range of 50-500. In this article, a new formation of CST is proposed for computing the electric field in short positively stressed rod-to-plane gaps characterized by G in the range of 2-160 and gap spacing up to 8 cm, provided that the gap can accommodate space charges. This is a prerequisite for computing the inception voltage of different corona modes, including burst pulses (BP), onset streamers (OS), and positive glow (PG). The criteria developed before for computing the inception voltage of BP and OS are refined for short rod-to-plane gaps. However, the inception criterion for PG cannot be extended to short gaps. A method is proposed for calculating the inception voltage of PG in short rod-to-plane gaps in the air at room temperature. The initiatory electrons for burst avalanches are produced by photoionization inside the ionization zone rather than by detachment of negative ions as experienced in long gaps. The resulting BP merges in time to form the steady glow at the rod surface in conformity with Loeb's postulate. The computed inception voltage of BP, OS, and PG agreed reasonably with those measured experimentally for short rod-to-plane gaps.