Any helpful pointers on how best to proceed with this one are
gratefully received.....

I've inherited a worksheet that is being used to calculate run times
for a real-world process. The for each object that going through the
process there is:

Stage1 - which takes a known period of time
Stage2 - a pause step that is user defined
Stage3 - which takes a known period of time

Currently, the total time is calculated by cutting and pasting the
various timings of each stage and adding them together - nice and
simple and for an indiviual object going through the process it's
okay. However, multiple objects are beginning to go through the
process and calculating the total run time in this manner is
becoming a nightmare. The reason being that if the Stage2 pause
step for the first object is sufficiently long, it's possible to
start the Stage1 step for the second object during this time before
having to go back and perform Stage3 on the first object. This means
that the total time isn't simply double the time for one object, it's
shorter by the degree parallel processing. It becomes even more
horrendous when a total of 50 objects have to go through the process,
so what I'd ideally like to do is simply present a userform to the
users asking how many objects they want to go through the process
and what the length of the pause is. Some coding behind the form
should then be able to figure out how many objects can be run in
parallel, and then dump the total process time into the worksheet.

What I can't figure out is a decent way of doing the maths/logic or
whatever you want to call it. Any ideas?