There was no need to be that precise, I just decided to try and do the maths and get it as close as I could.
In the end I got 50.0xxHz on the oscilloscope.
The question about trim pots is more general as they seem to be quite common for anything you want precision trimming on.
As I understand it you can set the minimum and maximum of the circuit via parallel and series calculations. It's that bit that is a bit of a faff to work out, the only way I have found (as I'm loosy with anything but basic algebra) is trial and error in a parallel / series online calculator.
+------R220--------+
-| |-
+-R220---T0-5K---+
Gives you from 220/220 in parallel to 220/5220 in parallel. 110 Ohms - 211 Ohms across the full sweep of the trim pot, instead of 5K which is useless for trimming.
I'm pretty sure a calculator that would take the 110 Ohms and 211 Ohms and result in one or more possible combinations would be extremely useful. Would be easier if you specify the trim pot size.
Note it could be something like 4K - 5.4K to trim a 4k7 by +-500Ohms. Doesn't have to be for precision.