looking up data from other tables

Hi,

I am trying to improve a current data base that is not been constructed brilliantly in the first place but working with it.

I have a form linked to a table (I am adding a second table to it) which has lots of data as massive tables full of time points and samples at specific time points.

I am creating a subsequent form that will pull certain data from the above form and table(s) and letting me add some at bits to produce a report.

One of the things that I want to add is looking up how many samples have been taken up to a certain time point and how many are left. But would prefer not to do the maths and can lead to error...

There can be up to 6 samples on each project, and will be 15 time points.
But not to sure how to specify which time point that I am using in the subsequent form to link to the correct field in the original table(s)

Anybody got any ideas?
been thinking of iif/lookups but not really too sure how these work tbh.

Thanks
Mikey

looking up data from other tables