It's easy enough to calculate. Get to 70 (or whatever) in 5th. Note your rpm.
Divide that RPM by the speed, so RPM/70 for this example. Then multiply the result by the speed you would like to know the rpm for.
Using your original example:
4500rpm/125mph=36rpm/mph.
If you want to know how many rpm you will be doing at 70 in the same gear, just do 36*70=2520rpm.
Looks like you have got your figures wrong, because 70 in 5th should be about 2000rpm. If I use your "mite of been 3500rpm" comment, then it comes out to 1960rpm which sounds more like it.
On a perhaps more important note, where are you doing 125mph and still running out of road before getting into 6th gear?? You shouldn't be doing this anywhere except for a test track but from the sound of it you must be doing it on a country road/A road/dual carriageway?
Let me know where you drive and I will do my best to stay clear.
Only last week some bloke in a boyed-up Clio crashed into a Focus pulling out of the end of my road. He admitted to doing at least 40 (so probably 50+) in a 30 zone. His girlfriend hit her head on the windscreen and shattered it. How nice for her........
Sorry, but doing those sorts of speeds on the public roads is just plain stupid.