There is no average to the amount of drop points on any round, I mean there obviously is in a mathematical sense, but not one that is transferable to work.
All walking duties whether full time or part time should have a maximum (at the moment) delivery span of 3.5 hours. The factors that affect the amount of DPs are:
Terrain.
Housing Density.
Access distance (how long a path or drive is up to the actual DP)
Location of Safe Drops and Pouch Boxes
Method of delivery (Cycle, HCT, OMV, Foot all have speeds that can be set up in Pegasus)
Average attendance calls - (How many times you have customer interaction on delivery)
Weight of bags as per matrix
Call Rate
Travelling time (whether or not you are accelerated out) to and from the duty.
If the postie on the reduced duty has been told he can go down to 20 hours this will mean he will do 4 hours per day(including breaks). So unless the duty is pre-prepped for him and all he has to do is pick up bags and go out, then they will have to reduce the amount of DPs or pay him OT. Therefore if he has to prep and deliver all within 4 hours then yes they will have to reduce the amount of deliveries he has to make.
----------------------------------------
D2Ds - The agreement on D2Ds is that 20% has to be done everyday Mon - Fri. So yes the DIM can force you to do it if he wants to be petty. However, most sensible managers would turn a blind eye to who is actually doing them as long as they are completed by the end of the week. However, in line with the agreement you should be doing 20% on each walk you cover, and if you don't you are running the risk of being disciplined for delay of mail. As they say if you know the rules and understand the consequences and are willing to take the chance then go for it and hope that the duty holder does his bit for you.