Sorry for the delayed response - I have been away.
Roganjosh is correct in that I was looking for a dynamic solution, however following his advice and rethinking that actual need, realised that it was not necessary for our situation. I was looking to adjust the pickup time for multiple jobs at the same location at the same time by the same vehicle with a fixed fleet of about 5 vehicles. The suggestions to adjust the travel time do work technically but do not work (for us) from a "business outcomes" POV because it obscures information important to us in gaining an ongoing time/motion understanding of the business.
Everything I looked at for dynamic calculation proved either unrealistic or just too hard.
So instead of calculating time on site as a factor of the number of items being loaded we did it as a factor of weight. For our environment that was actually a better outcome and did not need to be calculated dynamically.
We set a nominal time in seconds per kg loaded (typically about 30 parcels each of a range around 10kg onto each truck). We then set the TOS at pickup as the job is added to the problem set using (weight * rate).
Early testing is showing this to, ostensibly, be a realistic estimation model. A benefit of this approach is that the TOS variance caused by heavier items is automatically allowed for.
Production testing mode in a few weeks will be the proof of the pudding.
If you need some help considering models/methods then post a bit of a synopsis of your situation. I am not an expert but am willing to ponder it.