So, every time someone fully attends a Seminar, they get 100 for that activity, and if they are marked "Did not attend" they get a 0. The distinction between that "Did not attend" and them not being booked on is that someone who isn't booked on doesn't get a mark for the activity at all, which will be important later.

So, as you say, you want to calculate where people have attended 70% of the sessions, which means changing the way the overall course grade is calculated from "Natural" (add all of the grades up, scaled by weight) to "Mean of grades" (add all of the grades up and divide by the number of grades). There is also an option in the Gradebook setup setting for the course whether to "Exclude empty grades" and you want to make sure that's unticked so that sessions where staff aren't booked still get counted. There is also as "Drop the lowest" option, and this should be 0.

So, an example:

• There are 4 Seminar activities, A B C and D.
  
• The course completion setting is requiring a grade of 70.
• User attends Seminar A. Their grades are therefore A = 100, B = , C = , D = and the course total is therefore 100 / 4 which is 25. 
• User then attends B but doesn't turn up to their booking for C. Their grades are now A = 100, B = 100, C = 0, D = and the course total is therefore 200 / 4 which is 50.
• User then rebooks for another session of activity C, and this time turns up. The grades are now A = 100, B = 100, C = 100, D = which means a course total of 300 / 4 which is 75 and the course is marked as complete.

The downside is, of course, that you have to create 46 separate Seminar activities, each with one date in (assuming you only run them once), but it should work.