Tixati 3.19 linux.

I'm trying to find the "sweet spot" for download and upload speeds...

My "normal" internet connection speed tests (no VPN) are consistent 300 Mbps down and 12 Mbps up. If I have converted correctly, that's 37500 KBps down and 1500 KBps up.

I use Tixati over a VPN, and speed tests are consistently a bit over 5 Mbps down and up. I set Tixati to 600 KB/s down and up and that seems to work OK.

Afer reading the optimizing guide, I unconstrained the download speed and set the upload to 200 KB/s. This seems to be fine too, but what confuses me is Tixati shows downloading speed consistently at 1.2 MBps, or 9.6 Mbps, when the connection is supposed to only support 5Mbps?

In VPN settings, split tunnel is turned off, and LAN traffic is not permitted to bypass the VPN. If the VPN drops, the kill switch is absolute anad no traffic leavves the box. I've tried a handful of network analysis tools, all seems to indicate the VPN is working correctly and is not leaky.

So how do I reconcile these different figures and come up with the best optimized settings? Or are my calculations wrong?

First of all you should go to Tixati's settings and make sure the Connections -> Local IPv4/IPv6 address or interface is set to that of the VPN interface. Tixati will respect this and restrict itself only to the VPN at all times.
Secondly the issue is with the VPN connection itself. I'm assuming you are connecting to a nearby server so the most likely cause of low throughput is an issue with MTU. Surely go ahead and try other servers first but if they are roughly the same, try connecting to IPv6 first and then begin experimenting with lower MTU values of the VPN connection. This is a very painful procedure and not always a success.

Thanks for the suggestions. I have done or tried everything you mentioned plus a few more things, I just can't explain how speedtests to lots and lots of servers near and far (including one physically located near VPN server) could consistently deliver the lower download speed while Tixati can download at so much higher speed. I still cannot find any leaking.

Tixati runs hundreds of connections while speed tests attempt around a dozen parallel connections at best. What you describe there's still a fundamental slowdown for each single connection but the way Bittorrent works, Tixati just doesn't care much. I've had that issue with my VPN where Tixati would eventually reach 100% speed, but some protocols/apps could not. In my case I found out it was a network-level issue somewhere between my ISP and the VPN's data center network. Switching servers had helped.