The 377/5 run 20 of FCC 99 daily diagrams so roughly 20% of all FCC services are 377. The St Pancras - Three Bridges is the only 4 car service from Thameslink down the Brighton mainline via Redhill, hence it being empty at St Pancras (the other being an 8 car 319 Brighton service).
Just to give you an idea, off peak FCC run 10 services per hour through the Core.
4 Bedford - Brighton
2 Loop - St Albans/Luton stoppers
2 Kentish Town - Sevenoaks
Using 20% figure half the Bedford - Brighton trains would 377 off peak. Tho remember in peak the extra Rochester/Ashford service reduces the 377 Brighton service in PM peak.
The 319 on the 17:40 St Pancras to Three Bridges is non-standing all the way to Three Bridges (well at least Redhill anyway). It is one of the least used rush hour services from London, although the 8 car 377/5 Ashford one just in front of it is also often fairly empty at St Pancras but I don't know if that fills up before Blackfriars.
There are 4 loop services per hour that add up to your 10, so only half the Bed- Brighton trains off -peak could be 377's. Then with the Rochester/Ashford services they use up the 377's in the rush hour.
Why did FCC decide to choose the Ashford route for the new trains over its existing customers on the Brighton route?
By the way - we have posters from FCC heralding their 23 new trains on Redhill station - ironic as we never see any on our services - bet the Wimbledon Loop has plenty of the posters as well.