There is no straightforward advice on the use of resolution enhancement, other than to try 0.5 a bit at a time.
Section 220.127.116.11 of PS6 user guide and the following link http://www.picotech.com/education/oscil ... ering.html
provide an explanation of trigger hysteresis and, in the case of the PS2203, the hysteresis is currently fixed at 10 LSB of the selected range.
As suggested previously, you may wish to consider a unit such as the PS2205 which allows advanced edge triggers in which you may define your own hysteresis levels.
With reference to your final comment, as your probe allows PicoScope to manage the range selection automatically, any such effects would be minimised.
The trigger level remains in volts as that does not change in practice. Nonetheless, the scaled value may be determined using a marker on the trigger diamond.