All teaching staff whether permanent or fixed term, will use the same number of banking staffing fttes if employed over the same period of time. If a whole year, and employed full time for example, each will use 26 fttes.
It follows that if you are looking to offset overuse now to year end and need a person transferred to bg until pp 22 you can use the cheapest salary available, regardless of tenure providing you wish to keep them in bg until pp 22.
Most will reduce your banking staffing usage by 1 ftte per fortnight as you would expect, but some who will be paid up to year end in pp 19 will reduce your overuse by about 3 or 4 fttes in pp 19, then have no influence in pp 20, 21 and 22.
The only difference is timing, so there is no problem with using people who finish at year end, just that pp 19 usage can look very odd when the temporary effect of their usage dramatically alters the anticipated balance at pp 19. Remember they will be off the radar for the rest of the year and the situation will right itself.
So use pp 18 to decide who to put in bg based on salary rates and do not respond to any wild swings in the pp 19 banking staffing report.
The other side of this discussion is to suggest you avoid putting someone who is leaving at year in bg for pp 19 if you are only wanting to claw back say 1 ftte. Their accumulated entitlement will probably be 3 or 4 fttes which would all be processed in pp19 of course!