Hi Keith,
I guess the first fuzzy bit I have is understanding what a TS-420 is. I've not heard of that rig so I am unable to offer any tips about that specific radio.
Here's a GREAT link for all FLDigi users:
https://orcadigitalnet.com/quick-start/In a nut shell. ALL digital modes are in USB mode. Many new ops don't realize this as their radios will default to LSB since that is the typical mode for 75/80 meters. So make sure your rig is in USB while you're enjoying the digital modes.
Since this is an old radio, you will likely have to listen for callers and do your best to center on their frequency on the FLDigi water fall. It's especially easy if the ops use the "TUNE" button on FLDigi, you will see that and can easily click on that signal in the water fall with your mouse.
The sweet spot was chosen over a decade ago as being the default in FLDigi. Very rarely do I see stations not using the 1500hz. Now a caveat is working PSK31 mode. This is an ultra-narrow mode where many ops can use a single frequency (on radio dial) and simply click on any given audio signal on the waterfall to QSO with.
Another very handy tip is make sure to turn on the RxID at the top right of FLDigi. If an op is transmitting a TxID your station should hear that ID and automatically change into the correct operating mode the sending station is using.
If your radio drifts, you will have to compensate for it manually in FLDigi. You can use the arrows on either side of the water fall frequency displayed center bottom of FLDigi screen. However, if you are using PSK31 or any of the PSK modes, you can turn on the AFC button at bottom right and this should track that signal as you qso. Other modes, you will want to turn off the AFC.
I hope you enjoy the modes!
73
Jaye